- Email: [email protected]
Positive Regulation of the Nuclear Activator CREM by the Mitogen-Induced p70 S6 Kinase
Immunbiol., vol. 193, pp. 155-160 (1995)
©
1995 by Gustav Fischer Verlag, Stuttgart
1 Laboratoire
Genetique Moleculaire des Eucaryotes, Faculte de Medecine, Institut de Chimie Biologique, Strasbourg, France, and 2Research Institute of Molecular Pathology, Vienna, Austria
Positive Regulation of the Nuclear Activator CREM by the Mitogen-Induced p70 56 Kinase ROLF P.
DE GROOT
I::., LISA M. BALLOU 2 , and PAOLO SASSONE-CORSI I
The CREM gene Transcriptional regulation upon stimulation of the adenyl ate cyclase pathway is mediated by the family of cAMP response element (CRE)-binding proteins (reviewed in reference 1). This family consists of a large number of transcription factors containing basic domain/leucine zipper (bZip) motifs (reviewed in reference 2). We have recently cloned a novel member of the CRE-binding protein family, CREM (CRE modulator). The CREM gene encodes a large number of different proteins with distinct functions. CREM a, ~ and y encode transcriptional repressors (3), while CREM also encodes transcriptional activators (CREM-.;, -.;1 and -.;2 (4). These different proteins are generated by alternative splicing. Another mechanism by which the CREM gene generates multiple protein products is alternative promoter usage. While all the above mentioned CREM transcripts are generated by transcriptional initiation at an upstream promoter (Pl), the CREM gene also contains an intronic promoter (P2). Initiation at this promoter generates multiple small CREM transcripts consisting mainly of the bZip region that act as transcriptional repressors (5, 6). Since P2 is rapidly induced upon cAMP signalling due to the presence of multiple CRE sequences, these isoforms were named ICER (inducible cAMP early repressor; 6). The induction of ICER is thought to be important for the transient nature of cAMP-induced gene expression.
:,. Present address: Department of Pulmonary Diseases, University Hospital Utrecht,
PO Box 85500, 3508 GA Utrecht, The Netherlands
156 . R. P.
DE GROOT,
L. M.
BALLOU,
and P.
SASSONE-CORSI
Adivation by phosphorylation CREMt, like CREB, is rapidly phosphorylated on serine 117 by PKA upon cAMP signalling, and is therefore implicated in cAMP-induced gene expression (7). In addition, CREM and CREB are also nuclear targets for the Ca2 + I calmodulin -dependent signalling pathway. Stimulation of this pathway by membrane depolarization or by treatment with a Ca2 + ionophore leads to rapid phosphorylation of CREB and CREMt on the serine residue that is also phosphorylated by PKA (7). Both proteins can be efficiently phosphorylated on this residue by Ca2 +Icalmodulin-dependent kinases (CamK) in vitro (7). In addition, trans-activation by CREB and CREMe is strongly enhanced by this pathway. Similarly, CREMe activity can be regulated by the phorbol-ester-induced PKC pathway (7). CREM is also an efficient substrate for casein kinase (CK) I and II. Phosphorylation by these enzymes occurs at multiple sites, and is implicated in the regulation of the DNA-binding capacity of CREM (7). CREM activity can also be negatively regulated by phosphorylation. We have previously demonstrated that p34 cdc2 can phosphorylate CREMe on four different residues 'in vitro, of which only two are good consensus sequences for this kinase (8). In cycling COS cells overexpressing CREM, all four of these residues are phosphorylated. Phosphorylation by p34 cdc2 did not alter the DNA-binding capacity of CREMe. Br contrast, cotransfection with a constitutively active mutant of p34 c c2 significantly reduced the transcription activation potential of CREMe, suggesting that CREMe might be negatively regulated by p34 cdc2 in vivo (8). This might be important for the regulation of CRE-containing genes during different stages of the cell cycle.
Mitogenic stimulation adivates CREM-I: Gene regulation by polypeptide growth factors is thought to be mediated by transcription factors that are controlled by the map-kinase pathway. However, polypeptide growth factors also can activate a parallel but distinct signalling pathway, leading to activation of the p70 S6 kinase (p70 S6k ). This kinase will rapidly phosphorylate the S6 protein of the 40S ribosomal subunit upon mitogenic stimulation. The importance of p70s6k activation for G 1-5 transition was demonstrated by antibody-injection experiments. Moreover, inhibition of this enzlme by the immunosuppressant rapamycin also demonstrates that p70 s6 plays an important role in mitogenic stimulation. However, no nuclear targets for p70 S6k that are involved in transcriptional regulation have been identified so far. Recently, we have demonstrated that treatment of cells with fetal calf serum (FC5) will also enhance CREMe phosphorylation (9). It was shown that serine 117 is the target for this FC5-induced phosphorylation. Cotransfection experiments showed that trans-activation by CREMe was
CREM and p70 S6 kinase . 157
significantly enhanced by FCS treatment, this effect being dependent on serine 117 (9). In an attempt to identify the responsible kinase, we have tested a number of FCS-induced kinases on CREMt in vitro. While both MAP kinase and pp90 RSK (risk SG kinase) failed to phosphorylate CREM"t, the mitogen-activated p70s6k proved to be a very efficient CREM"t kinase, phosphorylating CREM"t solely on serine 117 (9). The Km and Vmax for CREM"t phosphorylation were comparable to the previously published values for 40S ribosomal S6 protein, suggesting that CREM"t might be a valid substrate for p70 S6k . p70 S6k is thought to reside primarily in the cytoplasm, while CREM is a nuclear protein. Therefore, we studied the sub-cellular localization of p70S6k more carefully. Cellular fractionation studies demonstrated that there is considerable p70 S6k protein and activity present in the nucleus (9). Moreover, immunofluorescence on COS cells transfected with a p70 S6k expression vector confirmed the presence of p70S6k in the nucleus. Therefore, p70 s6k might be involved in phosphorylation of CREM"t in vivo. To determine the functional consequence of CREM phosphorylation by p70 s6k , we studied DNA binding and trans-activation by CREM"t. While we failed to detect an effect of phosphorylation by p70 s6k on DNA binding by CREM"t, its trans-activation potential was strongly enhanced by co-transfection with a p70s6k expression vector (9). Moreover, transfection of p70 S6k and treatment with FCS cooperatively enhanced trans-activation by CREM"t, which correlated with enhanced p70 S6k activity as determined by in vitro immuno-kinase assays. To prove that p70 S6k was indeed responsible for the observed effects of FCS treatment on CREM"t phosphorylation and trans-activation, cells were treated with the immunosuppressant rapamycin. This drug will selectively
A
S6K FCS Rapamycin
++ + +
++ ++ ++ + +
Figure 1. Rapamycin inhibits phosphorylation of CREMt by p70 S6k • A: p70 S6k was immunoprecipitated from serum-stimulated untransfected COS cells or COS cells transfected by the p70 s6k expression vector. Immunoprecipitated p70 S6k was used to phosphorylate CREMt or ribosomal S6 protein in vitro. Rapamycin strongly decreases both the basal level and the serum-induced p70 S6k activity. - B: Rapamycin blocks FCS-induced phosphorylation of CREMt. COS cells were transfected with CREMt or CREMt-117 (Ser 117 mutated to Ala), labeled with 32p-orthophosphate and serum-stimulated as in Figure A. Rapamycin blocks FCS-induced phosphorylation of CREMt as well as decreases the basal level phosphorylation of CREMt, but not of CREMt-117.
158 . R. P. DE GROOT, L. M. BALLOU, and P. SASSONE-CORSI
block activation of p70 S6k by FCS, while activation of MAP kinase and pp90 RSK are not influenced. Rapamycin treatment indeed blocked FCS-induced phosphorylation of CREM"C, both in vitro as well as in vivo (Fig. 1 A and B). Moreover, both the basal level as well as the FCS-induced transactivation by CREM"C were severely impaired by rapamycin (Fig. 2), while transactivation by an unrelated protein (VP16) was not affected. These results clearly demonstrate that p70 S6k is a positive regulator of CREM"C function in vivo (9).
CREMt is a target for cross-talk between different signal transduction pathways The above mentioned results show that both CREM"C and CREB can act as nuclear targets for multiple phosphorylation cascades that can cross-talk. These transcription factors will therefore be able to induce CRE-dependent gene expression after stimulation of different signal transduction pathways (Fig. 3). This is exemplified by the fact that the CRE present in the c-fos promoter is necessary for c-fos induction by the cAMP pathway as well as the ci+ -dependent pathway. Similarly, a CRE/TRE-like sequence in the c-jun promoter is implicated in c-jun induction by TPA as well as by serum growth factors. Finally, the CRE present in the fibronectin promoter was demonstrated to be inducible by cAMP as well as by serum, which fits nicely with our results showing that the serum-inducible p70 S6k will phosphorylate and activate CREM"C (9). These cross-talk phenomena can also work reciprocally. We and others have previously demonstrated that TP A -induced TRE activation is enhanced by increased cAMP levels, and TRE-dependent trans-activation by
Rapamycin S6K
FeS
+
+f+f
+ +i+i+
+ + + + + +
Figure 2. Rapamycin blocks FCS- and p70s6k-induced activation of CREM't. Swiss 3T3 cells were transfected with G4CREM't, G4CREM't-117, G4VP16, the G4CAT reporter and the p70 S6k expression vector. Four hours before harvesting, some samples were treated with FCS. Rapamycin clearly inhibits FCS/p70 s6k -induced trans-activation by CREMT, while the activity of G4VP16 was not altered.
CREM and p70 S6 kinase . 159 KCI
•
C3 --- ~8
~
-8
! REPRESSION
- - - ......
..
ACTIVATION
8
Figure 3. CREB and CREM are targets for cross-talk between multiple signalling pathways. Activation of adenylate cyclase (AC) through f or example the ~ adrenergic receptor (~AR) will lead to activation of protein kinase A (PKA) and phosphorylation of CREB and CREM. These proteins can also be activated by Ca2+ Icalmodulin-dependent kinases (CamK) after membrane depolarization (KC1). Calcineurin (CN) seems to be involved in this pathway . Similarly, activation of protein kinase C (PKC) or the growth factor (GF)-induced p70 S 6 kinase will also lead to phosphorylation and activation of CREB and CREMT. C RE-d ependent transcription can also be activated by dimerization of CREB-like proteins with Jun, whose activity is regulated b y the MAP kinase (MAPK) pathway . Casein kinase (C K) Tand II enhance CRE binding by CREB and CREM, while simultaneously repressing TRE binding by Jun. CRE-dependent transcription can be negativel y regulated by protein phosphatases (PP) 1 and 2A, by p34 cdc2 and by induction of CREM-ICER proteins. Further points of rcoss-talk are: repression of the raf kinase by PKA; repression of TRE-dependent transactivation by CREB and CREM; and activation of Jun fun ction b y PKA. SOS = son of sevenless; MAPKK = MAP kinasekinase.
Jun/ AP-l can be strongly enhanced by the catalytic subunit of PKA (Fig. 3). On the other hand, cAMP strongly represses the activity of the raf kinase (Fig. 3). The possibilities for cross-talk are further increased by the potential of certain CRE-binding proteins to dimerize with the cJun protein, whose activity can also be regulated by multiple signalling pathways (Fig. 3). Moreover, we have previously shown that CREB and CREM can bind to TRE sequences, thereby strongly inhibiting their activity (Fig. 3). In conclusion, in this way alimited number of transcription factors can be responsible for the diverse changes in gene expression induced by multiple signalling pathways.
160 . R. P. DE GROOT, L. M. BALLOU, and P. SASSONE-CORSI
References 1. GROOT, R. P. DE, and P. SASSONE-CORSI. 1993. Mol. Endocrinol. 7: 145. 2. BUSCH, S. J., and P. SASSONE-CORSI. 1990. Trends Genet. 64: 36. 3. FOULKES, N. S., E. BORRELLI, and P. SASSONE-CORSI. 1991. Cell. 64: 739. 4. FOULKES, N. 5., B. MEl.LSTROM, E. BENUSIGLIO, and P. SASSONE-CORSI. 1992. Nature 355: 80. 5. STEHLE, J. H., N. S. FOULKES, C. A. MOLINA, V. SIMONNEAUX, P. PEVET, and P. SASSONE-CORSI. 1993. Nature 365: 314. 6. MOLINA, C. A., N. S. FOULKES, E. LALLI, and P. SASSONE-CORSI. 1993. Cell 75: 875. 7. GROOT, R. P. UE, J. DEN HERTOG, J. R. VANDENHEEDE, J. GORIS, and P. SASSONECORSI. 1993. EMBO J. 12: 3903. 8. GROOT, R. P. DE, R. DERUA, J. GORIS, and P. SASSONE-CORSI. 1993. Mol. Endocr. 7: 1495. 9. GROOT, R. P. DE, L. M. BALLOU, and P. SASSONE-CORSI. 1994. Cell 79: 81.
Dr. PAOLO SASSONE-CORSI, Laboratoire Genetique Moleculaire des Eucaryotes, CNRS, U 184 de L'INSERM, Faculte de Medecine, Institut de Chimie Biologique, Strasbourg, France
©
1995 by Gustav Fischer Verlag, Stuttgart
1 Laboratoire
Genetique Moleculaire des Eucaryotes, Faculte de Medecine, Institut de Chimie Biologique, Strasbourg, France, and 2Research Institute of Molecular Pathology, Vienna, Austria
Positive Regulation of the Nuclear Activator CREM by the Mitogen-Induced p70 56 Kinase ROLF P.
DE GROOT
I::., LISA M. BALLOU 2 , and PAOLO SASSONE-CORSI I
The CREM gene Transcriptional regulation upon stimulation of the adenyl ate cyclase pathway is mediated by the family of cAMP response element (CRE)-binding proteins (reviewed in reference 1). This family consists of a large number of transcription factors containing basic domain/leucine zipper (bZip) motifs (reviewed in reference 2). We have recently cloned a novel member of the CRE-binding protein family, CREM (CRE modulator). The CREM gene encodes a large number of different proteins with distinct functions. CREM a, ~ and y encode transcriptional repressors (3), while CREM also encodes transcriptional activators (CREM-.;, -.;1 and -.;2 (4). These different proteins are generated by alternative splicing. Another mechanism by which the CREM gene generates multiple protein products is alternative promoter usage. While all the above mentioned CREM transcripts are generated by transcriptional initiation at an upstream promoter (Pl), the CREM gene also contains an intronic promoter (P2). Initiation at this promoter generates multiple small CREM transcripts consisting mainly of the bZip region that act as transcriptional repressors (5, 6). Since P2 is rapidly induced upon cAMP signalling due to the presence of multiple CRE sequences, these isoforms were named ICER (inducible cAMP early repressor; 6). The induction of ICER is thought to be important for the transient nature of cAMP-induced gene expression.
:,. Present address: Department of Pulmonary Diseases, University Hospital Utrecht,
PO Box 85500, 3508 GA Utrecht, The Netherlands
156 . R. P.
DE GROOT,
L. M.
BALLOU,
and P.
SASSONE-CORSI
Adivation by phosphorylation CREMt, like CREB, is rapidly phosphorylated on serine 117 by PKA upon cAMP signalling, and is therefore implicated in cAMP-induced gene expression (7). In addition, CREM and CREB are also nuclear targets for the Ca2 + I calmodulin -dependent signalling pathway. Stimulation of this pathway by membrane depolarization or by treatment with a Ca2 + ionophore leads to rapid phosphorylation of CREB and CREMt on the serine residue that is also phosphorylated by PKA (7). Both proteins can be efficiently phosphorylated on this residue by Ca2 +Icalmodulin-dependent kinases (CamK) in vitro (7). In addition, trans-activation by CREB and CREMe is strongly enhanced by this pathway. Similarly, CREMe activity can be regulated by the phorbol-ester-induced PKC pathway (7). CREM is also an efficient substrate for casein kinase (CK) I and II. Phosphorylation by these enzymes occurs at multiple sites, and is implicated in the regulation of the DNA-binding capacity of CREM (7). CREM activity can also be negatively regulated by phosphorylation. We have previously demonstrated that p34 cdc2 can phosphorylate CREMe on four different residues 'in vitro, of which only two are good consensus sequences for this kinase (8). In cycling COS cells overexpressing CREM, all four of these residues are phosphorylated. Phosphorylation by p34 cdc2 did not alter the DNA-binding capacity of CREMe. Br contrast, cotransfection with a constitutively active mutant of p34 c c2 significantly reduced the transcription activation potential of CREMe, suggesting that CREMe might be negatively regulated by p34 cdc2 in vivo (8). This might be important for the regulation of CRE-containing genes during different stages of the cell cycle.
Mitogenic stimulation adivates CREM-I: Gene regulation by polypeptide growth factors is thought to be mediated by transcription factors that are controlled by the map-kinase pathway. However, polypeptide growth factors also can activate a parallel but distinct signalling pathway, leading to activation of the p70 S6 kinase (p70 S6k ). This kinase will rapidly phosphorylate the S6 protein of the 40S ribosomal subunit upon mitogenic stimulation. The importance of p70s6k activation for G 1-5 transition was demonstrated by antibody-injection experiments. Moreover, inhibition of this enzlme by the immunosuppressant rapamycin also demonstrates that p70 s6 plays an important role in mitogenic stimulation. However, no nuclear targets for p70 S6k that are involved in transcriptional regulation have been identified so far. Recently, we have demonstrated that treatment of cells with fetal calf serum (FC5) will also enhance CREMe phosphorylation (9). It was shown that serine 117 is the target for this FC5-induced phosphorylation. Cotransfection experiments showed that trans-activation by CREMe was
CREM and p70 S6 kinase . 157
significantly enhanced by FCS treatment, this effect being dependent on serine 117 (9). In an attempt to identify the responsible kinase, we have tested a number of FCS-induced kinases on CREMt in vitro. While both MAP kinase and pp90 RSK (risk SG kinase) failed to phosphorylate CREM"t, the mitogen-activated p70s6k proved to be a very efficient CREM"t kinase, phosphorylating CREM"t solely on serine 117 (9). The Km and Vmax for CREM"t phosphorylation were comparable to the previously published values for 40S ribosomal S6 protein, suggesting that CREM"t might be a valid substrate for p70 S6k . p70 S6k is thought to reside primarily in the cytoplasm, while CREM is a nuclear protein. Therefore, we studied the sub-cellular localization of p70S6k more carefully. Cellular fractionation studies demonstrated that there is considerable p70 S6k protein and activity present in the nucleus (9). Moreover, immunofluorescence on COS cells transfected with a p70 S6k expression vector confirmed the presence of p70S6k in the nucleus. Therefore, p70 s6k might be involved in phosphorylation of CREM"t in vivo. To determine the functional consequence of CREM phosphorylation by p70 s6k , we studied DNA binding and trans-activation by CREM"t. While we failed to detect an effect of phosphorylation by p70 s6k on DNA binding by CREM"t, its trans-activation potential was strongly enhanced by co-transfection with a p70s6k expression vector (9). Moreover, transfection of p70 S6k and treatment with FCS cooperatively enhanced trans-activation by CREM"t, which correlated with enhanced p70 S6k activity as determined by in vitro immuno-kinase assays. To prove that p70 S6k was indeed responsible for the observed effects of FCS treatment on CREM"t phosphorylation and trans-activation, cells were treated with the immunosuppressant rapamycin. This drug will selectively
A
S6K FCS Rapamycin
++ + +
++ ++ ++ + +
Figure 1. Rapamycin inhibits phosphorylation of CREMt by p70 S6k • A: p70 S6k was immunoprecipitated from serum-stimulated untransfected COS cells or COS cells transfected by the p70 s6k expression vector. Immunoprecipitated p70 S6k was used to phosphorylate CREMt or ribosomal S6 protein in vitro. Rapamycin strongly decreases both the basal level and the serum-induced p70 S6k activity. - B: Rapamycin blocks FCS-induced phosphorylation of CREMt. COS cells were transfected with CREMt or CREMt-117 (Ser 117 mutated to Ala), labeled with 32p-orthophosphate and serum-stimulated as in Figure A. Rapamycin blocks FCS-induced phosphorylation of CREMt as well as decreases the basal level phosphorylation of CREMt, but not of CREMt-117.
158 . R. P. DE GROOT, L. M. BALLOU, and P. SASSONE-CORSI
block activation of p70 S6k by FCS, while activation of MAP kinase and pp90 RSK are not influenced. Rapamycin treatment indeed blocked FCS-induced phosphorylation of CREM"C, both in vitro as well as in vivo (Fig. 1 A and B). Moreover, both the basal level as well as the FCS-induced transactivation by CREM"C were severely impaired by rapamycin (Fig. 2), while transactivation by an unrelated protein (VP16) was not affected. These results clearly demonstrate that p70 S6k is a positive regulator of CREM"C function in vivo (9).
CREMt is a target for cross-talk between different signal transduction pathways The above mentioned results show that both CREM"C and CREB can act as nuclear targets for multiple phosphorylation cascades that can cross-talk. These transcription factors will therefore be able to induce CRE-dependent gene expression after stimulation of different signal transduction pathways (Fig. 3). This is exemplified by the fact that the CRE present in the c-fos promoter is necessary for c-fos induction by the cAMP pathway as well as the ci+ -dependent pathway. Similarly, a CRE/TRE-like sequence in the c-jun promoter is implicated in c-jun induction by TPA as well as by serum growth factors. Finally, the CRE present in the fibronectin promoter was demonstrated to be inducible by cAMP as well as by serum, which fits nicely with our results showing that the serum-inducible p70 S6k will phosphorylate and activate CREM"C (9). These cross-talk phenomena can also work reciprocally. We and others have previously demonstrated that TP A -induced TRE activation is enhanced by increased cAMP levels, and TRE-dependent trans-activation by
Rapamycin S6K
FeS
+
+f+f
+ +i+i+
+ + + + + +
Figure 2. Rapamycin blocks FCS- and p70s6k-induced activation of CREM't. Swiss 3T3 cells were transfected with G4CREM't, G4CREM't-117, G4VP16, the G4CAT reporter and the p70 S6k expression vector. Four hours before harvesting, some samples were treated with FCS. Rapamycin clearly inhibits FCS/p70 s6k -induced trans-activation by CREMT, while the activity of G4VP16 was not altered.
CREM and p70 S6 kinase . 159 KCI
•
C3 --- ~8
~
-8
! REPRESSION
- - - ......
..
ACTIVATION
8
Figure 3. CREB and CREM are targets for cross-talk between multiple signalling pathways. Activation of adenylate cyclase (AC) through f or example the ~ adrenergic receptor (~AR) will lead to activation of protein kinase A (PKA) and phosphorylation of CREB and CREM. These proteins can also be activated by Ca2+ Icalmodulin-dependent kinases (CamK) after membrane depolarization (KC1). Calcineurin (CN) seems to be involved in this pathway . Similarly, activation of protein kinase C (PKC) or the growth factor (GF)-induced p70 S 6 kinase will also lead to phosphorylation and activation of CREB and CREMT. C RE-d ependent transcription can also be activated by dimerization of CREB-like proteins with Jun, whose activity is regulated b y the MAP kinase (MAPK) pathway . Casein kinase (C K) Tand II enhance CRE binding by CREB and CREM, while simultaneously repressing TRE binding by Jun. CRE-dependent transcription can be negativel y regulated by protein phosphatases (PP) 1 and 2A, by p34 cdc2 and by induction of CREM-ICER proteins. Further points of rcoss-talk are: repression of the raf kinase by PKA; repression of TRE-dependent transactivation by CREB and CREM; and activation of Jun fun ction b y PKA. SOS = son of sevenless; MAPKK = MAP kinasekinase.
Jun/ AP-l can be strongly enhanced by the catalytic subunit of PKA (Fig. 3). On the other hand, cAMP strongly represses the activity of the raf kinase (Fig. 3). The possibilities for cross-talk are further increased by the potential of certain CRE-binding proteins to dimerize with the cJun protein, whose activity can also be regulated by multiple signalling pathways (Fig. 3). Moreover, we have previously shown that CREB and CREM can bind to TRE sequences, thereby strongly inhibiting their activity (Fig. 3). In conclusion, in this way alimited number of transcription factors can be responsible for the diverse changes in gene expression induced by multiple signalling pathways.
160 . R. P. DE GROOT, L. M. BALLOU, and P. SASSONE-CORSI
References 1. GROOT, R. P. DE, and P. SASSONE-CORSI. 1993. Mol. Endocrinol. 7: 145. 2. BUSCH, S. J., and P. SASSONE-CORSI. 1990. Trends Genet. 64: 36. 3. FOULKES, N. S., E. BORRELLI, and P. SASSONE-CORSI. 1991. Cell. 64: 739. 4. FOULKES, N. 5., B. MEl.LSTROM, E. BENUSIGLIO, and P. SASSONE-CORSI. 1992. Nature 355: 80. 5. STEHLE, J. H., N. S. FOULKES, C. A. MOLINA, V. SIMONNEAUX, P. PEVET, and P. SASSONE-CORSI. 1993. Nature 365: 314. 6. MOLINA, C. A., N. S. FOULKES, E. LALLI, and P. SASSONE-CORSI. 1993. Cell 75: 875. 7. GROOT, R. P. UE, J. DEN HERTOG, J. R. VANDENHEEDE, J. GORIS, and P. SASSONECORSI. 1993. EMBO J. 12: 3903. 8. GROOT, R. P. DE, R. DERUA, J. GORIS, and P. SASSONE-CORSI. 1993. Mol. Endocr. 7: 1495. 9. GROOT, R. P. DE, L. M. BALLOU, and P. SASSONE-CORSI. 1994. Cell 79: 81.
Dr. PAOLO SASSONE-CORSI, Laboratoire Genetique Moleculaire des Eucaryotes, CNRS, U 184 de L'INSERM, Faculte de Medecine, Institut de Chimie Biologique, Strasbourg, France