- Email: [email protected]
The enhancer shift: a model to explain the developmental control of IgH gene expression in B-lineage cells
IMMUNOLOGY
TODAY
The enhancer shift: a model to explain the developmental control of IgH gene,, expression in B-lineage cells Velmurugesan Arulampdam, Laurel Eckhardt and Sven kttersson
IMMUNOLOGY
EP
TODAY
cll
cs
_____------
cy3 _--
W
C@b
CIQa _--
_____------
HS3a C IRI
. It%?
CE Ca 3’Ca ___----
HSl HSZ --I 4--w IR3 WY
3’E
HS3
H.!$4
--__
--
--__
--_.
HS3b I IR2
--_ HS4
+ IRl’
expression. In this context, there is mounting evidence that their
suggesting a function at later stages of B-cell developments. Sri@ contains a domain that is shared with other proteins (e.g. SW11 that belong to the SWI/SNF complex previously shown to remodc chmmatin’. Therefore, Bright may be part of the ma&ii that al fects chmmatin structure. Furt!temwre, becauac of ita late&age ea The IgH intron (Ep) enhancer and MARS Eh is locatedbetween the jH-genesegment cluster and the constant pression, its function may also tp important for CSR and/or lgl gene transcription following antigen activation. [Cc) coding sequences(Fig. 1). Ubiquitous and lymphoid-specific transcription factors have both beenshown to bind DNA sequences NF-t&X was initially identified as a MAR-binding CornpIe within EQ (reviewed in Ref. 1). The importance of EP in early B-cell found only in non-8 cells where the MARa were shown to rep~a development has beenclearly demonstrated in the 6 cells of knock- I$. activitys. The home&ox protein Cux/CDP may be a member c pm. cummun.); Cux/CD out mice h&mzygous for deletion of this element: VDj assembly the NF-pNR complex (R. Schcucmwnn, was severely impaired upstream of the mutated IgH allele in these was previously identified as a regulator of myeloki gene expressi cells’2. Earlier studies of a mouse pre-B-cell line suggested that Ep and a repressor of the sea urchin sperm-specific hlstone gene was also required for IgH transcription subsequent to V-gene asThis protein also represses Ep enhancer function, the postulated hmctlon of NF-@JR activity (R. S&euermann, per sembly. lg expression ceased in this cell line upon spontaneous deletion of an intronic region encompassing Ep (Ref. 3). h&restcommun.). These findings suggest a model by which element ingly, fusion of this pre_B-cell lie with a myeloma line yielded within the MARs mediate and/or conhol chmmatin remodelin hybrids that had reactivated the mutant allele. events in developiag B cells. effectson chromatin are of particular importance to their function.
consistent wtt
The 1 kb fragment initially identified as the t$ enhancerin mice includes two MARs that flank a core region with mirltiple twn-
Cls-rting elements 8t the Y-end of the IgH locus are critical for efficient exprusion of linked transgenes when intro- Sttwturc ofthc mouse /gH locus duced into mice5. Moreover, without the MARS, transgene expms- Although EP plays a role in endogenons IgH gene assembly an sion leve!s varied among individual Ew-transgenic mice, suggasting possibly also in regulating IgH gene transcription after assembly, that the transgenes had become susceptible to chromosome was noted soon after its .iiscuvery that it was not -tial for tb position effects. Recent experiments suggest that the MARs associ- exprewion of IgH genes hr Ig-secreting ceUa3Jo.It was alao noted ated with EP have the capacity to prompt the region surrounding that chromosome translocations of the pmto-oncogene c-myc often a transgene to assume shuctural characteristics of transcriptionresulted in its juxtaposition with the 3’ region of the IgH locus. Since ally active chmmatb9. However, the Ek enhancer, including c-ntyc/lgH chromosome hanslocations had already been causally the MARS, has not been shown to generate copy-dependent linked to de-regulated catye expression and to transfonoation of Igexpression of transgenic reporter genes, a function athibuted producing I:% this suggested that elements in the 3’ region of the to locus conhol retions (LCRs; see below), when inserted into the IgH locus were capableof disrupting normal wnyc expression”. mouse germ line. A search for additionat +_-:~-_ elements dowstream of the TWO proteins that bind the Ep MAR have recently been identiIgH iocw was initiated and resultni in the identification of an fied: Bright (B-cell regulator of IgH transcription) and NF-pNR. IgH 3’ enhancer, pmently designated HSli iRefs 12-14) Wig. 1). Bright can be induced in lipopolysaccharide (LPSI-activated B cells, Another enhancer was then mapped closer to Ca (HS3a). as were
scription factor binding sites’. The two MARs and the core enhancer
IMMUN-OLOGY
TODAY
2
P
IMMUNOLOGY
TODAY
EP
CP
CY
cf
CCt
3’LCR
The ,x,rt,llels that can be drawn betw~n the @-globin and the ,gH 3wR
suggest mr,ain similti,ies
in lbeir nxcha”isr
of gene Tr-
gulati”“. Haivevw, it is dearly widen, that careful cotidera,i”ns m”s, be taken befare the pm&e mechanisms d LCR funni”” b-2 ““amblg”n”sly
.Sigwd
ca”
N”r cwmp!.?, ree Ref. 60,. .‘uthoygb
many quertions remin “nanswered. I, is h”ped that hrrther study of the IgH LCR will n”, Ml,y reveal ylme Of ,be mecbanisnlr Ip g”ta,ing IgH erpreai”n.bu,mayafs”
pmvidcrhe ~WN
ing the genenclic pmrrrrer ““iqve m this laur,
hrshldy
including CSR and
rnmanc byprm”,a,i”n.
NOVEMBER
1997
IMMUNOLOGY
lODAY
40 Su. L.K. and Kadesch.T. (1990)Mol. , Staudt, L.M. and Len&o, M.J. (1991)Atw~a
Rnv hm~ttnl
9, 373-398
I :I Chm, J.,Tmunstme,M., Att, FW. c’lnl. (1993)lrrl Iwmrrmd5, 647-656 / , Wabl, M.R. and Burrows,P.D tlYR4)Prw NatI
Arnd So
U. S A 81.
!452-2455 1 Cockerill,PN (1990 N~rclc..Arrds Rrr II), 2643-2648 ; Jenuwein,T., Forrester,WC., Fem.mdezherrcm, LA. cl nl. fl997l Nntwc U%5,269-272 i Hemcher, RF., Kaplan,M.H., L&z, D.L cl RI (1995)Cntt~ Da 9,
Crll.
Biol.lt’ 2619-2624
41 Schultz,C.L. and Coffman,R L. (1991)Cwr Opbt.hsawrol.3,350-354 42 DameIs,G.A. and Lie&r, M.R. (1996)Gun. Top.M~cmbiol. fmmmrol. 217, 171-189 43 Jung,S., RaJewsky, K. and Radbmch,A. (1993)5&vce 259,984_9S7 44 Zhang, J., Bottam,A., Li, S., Stewart,V. and Alt, EW. (1993) EMBO/. 12,3529-3537 45 X”, L., Corham, B., Li, SC. etRI. (1993)Pmt. Nntl. Acad. SCI.
I.!. S. A. 90,
3705-3709
1
46 Arulampalam. V., Grant, P.,Paellinger,L. and Pettersson, S. (1995) Mol. Itrm”“rol32, 1369-1375 47 Busslinger,M. and Urbanek,t?(1995)CUR.Opin.Gnat. Dm. 5,595-601
IO Eckhardt,L.A. and Bimhtein,B.K. (19X5)Mol. Cell.Eiol.5,856-86E
321-330
!Q67-3062 ’ *7 Steger,D J,and Workman,J.L.(1996)Ewwys lg. 875-%4 Scheuemwm,R.H. and Chen, U. (1989)Gtws Den 3,1255-1266 j; ) Barbwas,A., Super&Fur@,C. and 6ussltnger.M. (1987)Cell50,347-359
48 Sba,WC., Lieu, H.C., Tuomanen,E.I. and Baltimore.D. (1995) CellSO, al. W%)
/. Immtmd.
I1 Neuberger,MS. and Calabi, F. (1983)Natun 305,240-243 I2 Pettersson, S., Cook, G.P.,Bruggemann,M., Williams,C.T. and
49 Snapper,C.M., Zelamwski, I?, Row, RR. rl la-191
Ueuberger, MS. (1990)Nnfw’ 344,165-168
50 Michaclsmt,J.S.,Singh, M., Snapper,CM. P, al. (19%) J. fmmusd.156,
I3 Lieberson,R., Cimtnini,S.L., Birshtein,B.K.and Eckhardt,L.A. (1991) Vuclcic AcrdsRcs.19.933-937
2826-2839
156,
I4 Dartavach,t?,Williams,C.T., Campbell,K., Pettersso”,S. and
51 Lindersw”. Y., Newath, M., Cmsr, D. and P&te”mon,S. (1997) Eur.J. Immusd.27,468-475
Ueuberger, MS. (1991)Eur.J.bswunol.21.14994504
52 Elenich,LA., Ford, C.S.and Dunnick, W.A. (1996)J. fmmunol.157,
I5 Matthias,P.and Baltimore,D. (1993)Mol. Cell.Biol.13, 1547-1553
176-182
I6 Madison,L. and Groudi”e8M. (1994)Gctrcs Drrr 8.2212-2226
53 Towns, T. and Behrtnger,R. f1990)TwmfsGovt. 6,219-223
17 Gi”nnini, S.L., Singh,M., Calv”, C.E. Ding, C. and Birshtein,B.K. 1993)/. Int1ttamd. 150, 177%17&l I8 Chauveau,C. and Copne,M. (1996)Nat. Gcad. 14.15-16 I9 Mills, EC., Hartndranath,N., Mitchell,M. and Max, E.E. /. Exp. Med. ,in press) I Chen,C. and Birshtein,6.K. (1997)/. Irtmmwml. 159,1310-1318
56 Milot, E.. Stmuboulis,J.,Trimborn,T. d al. (1996)Ccl1S7,lMll4 57 Fiering,S., Epncr.E., RobiMon, K. cf d. (1995)Cmn &v. 9,22C3-2213 51) Hug. B.A., WewJschmklt,R.L., Fierf”g,S. ef al. (1996)Mol. Ctfl. Bfd. 16,25l%2912
ll Cmnt, P.A.,Arulampalam,V., Ahrlund-Richter,L. and Pettersson, S. 1992)NucfcicAcidsRes.20,4401-4406 t.2 Grant, P.,Thompson,C.B.and Pettenscn,S. (1995)EM60 J.14.45014513 23 Atulampalam.V.,Grant, P.A.,bmuelsson,A., Lendahl,U. and Petteram, S. (1994)Eur.J.Itnma,tof.24, 1671-1677 U Grant, P.A.,Andersson,T., New&h, M.F. t’, nl. (1996)EMsO/. 15, 5691-6700 25 Cogne,M., Lansford,R., Bottam,A. etnl. (1994)Cell77,737-747 26 Lieberson,R., Ong, J.,Shi, X. and Eckhadt, L.A. (1995) EMftO/. 14, 6229-623a 27 Gregor,P.D. and Morrison,S.L. (19%) Mol. Cell.Biol.6, 19Ll3-1916 26 Arulampalam.V., Furebring.C., hmuelsson, A. ul RI.(1996) lrtt. Imslll#tol8, 1149-1157 29 Jenuwein,T. and Gmwhedl, R. (1991)Gws Dru. 5,932-943 30 Corcoran,L.M., Karvelas,M., Nossal,G.J.V.etRI. (1993)Got‘s Dnr 7, 57~582 31 Luo. Y.. Fujii. H.. Center, T. and Rwder, R.C. (1992)Cell71.231-241 32 Luo. Y. and Roeder,RG. (199%Mol. Cell.Biof.15.41154124 33 Gstaiger,M.. Cargiev, 0.. van Leeuwen,H., van der Vliet, t?and Schaffner,W. (19%) EMS0 J. 15,2781-2790 34 Strubin.M.. Newell,J.W.and Matthias,f! (1995)Cvll80,497-506 35 Kim, U., Qtn, X.F.,Gong,S. ct nl. (19961N~brre383, 542-547 36 Schubart,D.B.. Rolink,A., Kouo-Vilbois,M.H., Bottert,E and Matthias,P.(1996)Nfllun 383.53%542 37 Zwllling, S.. Annweiler,A. and Wirth, T. (1994) NaclncActdrRq22, 1655-1662 M PftsterwP., Annweiler,A., Ullmer,C.,Corcoran,L.M. a”4 Wirlh. T. (1994)EMBOJ. 13.1655-1663 39 petersson,I?.Grant. P.,Andersso”,P. rf nl. (1995)Jnunnsologisf 3, 146-151
NOVEMBER
I997
54 Wiigerde,M., Gnnveld. E and Fraser,f?(1995)Nalu1?377,209-213 55 Ellis,J..Tan-Un, K.C., Harper,A. rt al. (19%) EMBO J. 15,56%56S
59 Ofron. E.N., Arnold, H.H., Rtgbp P.W.and W&i, Il.). (1996)CcffS5,14 60 Martin, D.L. Fiering,S. and Gmudine,M. (1996)Cwr. O@. Gene&Dcv. 6,488495
TODAY
The enhancer shift: a model to explain the developmental control of IgH gene,, expression in B-lineage cells Velmurugesan Arulampdam, Laurel Eckhardt and Sven kttersson
IMMUNOLOGY
EP
TODAY
cll
cs
_____------
cy3 _--
W
C@b
CIQa _--
_____------
HS3a C IRI
. It%?
CE Ca 3’Ca ___----
HSl HSZ --I 4--w IR3 WY
3’E
HS3
H.!$4
--__
--
--__
--_.
HS3b I IR2
--_ HS4
+ IRl’
expression. In this context, there is mounting evidence that their
suggesting a function at later stages of B-cell developments. Sri@ contains a domain that is shared with other proteins (e.g. SW11 that belong to the SWI/SNF complex previously shown to remodc chmmatin’. Therefore, Bright may be part of the ma&ii that al fects chmmatin structure. Furt!temwre, becauac of ita late&age ea The IgH intron (Ep) enhancer and MARS Eh is locatedbetween the jH-genesegment cluster and the constant pression, its function may also tp important for CSR and/or lgl gene transcription following antigen activation. [Cc) coding sequences(Fig. 1). Ubiquitous and lymphoid-specific transcription factors have both beenshown to bind DNA sequences NF-t&X was initially identified as a MAR-binding CornpIe within EQ (reviewed in Ref. 1). The importance of EP in early B-cell found only in non-8 cells where the MARa were shown to rep~a development has beenclearly demonstrated in the 6 cells of knock- I$. activitys. The home&ox protein Cux/CDP may be a member c pm. cummun.); Cux/CD out mice h&mzygous for deletion of this element: VDj assembly the NF-pNR complex (R. Schcucmwnn, was severely impaired upstream of the mutated IgH allele in these was previously identified as a regulator of myeloki gene expressi cells’2. Earlier studies of a mouse pre-B-cell line suggested that Ep and a repressor of the sea urchin sperm-specific hlstone gene was also required for IgH transcription subsequent to V-gene asThis protein also represses Ep enhancer function, the postulated hmctlon of NF-@JR activity (R. S&euermann, per sembly. lg expression ceased in this cell line upon spontaneous deletion of an intronic region encompassing Ep (Ref. 3). h&restcommun.). These findings suggest a model by which element ingly, fusion of this pre_B-cell lie with a myeloma line yielded within the MARs mediate and/or conhol chmmatin remodelin hybrids that had reactivated the mutant allele. events in developiag B cells. effectson chromatin are of particular importance to their function.
consistent wtt
The 1 kb fragment initially identified as the t$ enhancerin mice includes two MARs that flank a core region with mirltiple twn-
Cls-rting elements 8t the Y-end of the IgH locus are critical for efficient exprusion of linked transgenes when intro- Sttwturc ofthc mouse /gH locus duced into mice5. Moreover, without the MARS, transgene expms- Although EP plays a role in endogenons IgH gene assembly an sion leve!s varied among individual Ew-transgenic mice, suggasting possibly also in regulating IgH gene transcription after assembly, that the transgenes had become susceptible to chromosome was noted soon after its .iiscuvery that it was not -tial for tb position effects. Recent experiments suggest that the MARs associ- exprewion of IgH genes hr Ig-secreting ceUa3Jo.It was alao noted ated with EP have the capacity to prompt the region surrounding that chromosome translocations of the pmto-oncogene c-myc often a transgene to assume shuctural characteristics of transcriptionresulted in its juxtaposition with the 3’ region of the IgH locus. Since ally active chmmatb9. However, the Ek enhancer, including c-ntyc/lgH chromosome hanslocations had already been causally the MARS, has not been shown to generate copy-dependent linked to de-regulated catye expression and to transfonoation of Igexpression of transgenic reporter genes, a function athibuted producing I:% this suggested that elements in the 3’ region of the to locus conhol retions (LCRs; see below), when inserted into the IgH locus were capableof disrupting normal wnyc expression”. mouse germ line. A search for additionat +_-:~-_ elements dowstream of the TWO proteins that bind the Ep MAR have recently been identiIgH iocw was initiated and resultni in the identification of an fied: Bright (B-cell regulator of IgH transcription) and NF-pNR. IgH 3’ enhancer, pmently designated HSli iRefs 12-14) Wig. 1). Bright can be induced in lipopolysaccharide (LPSI-activated B cells, Another enhancer was then mapped closer to Ca (HS3a). as were
scription factor binding sites’. The two MARs and the core enhancer
IMMUN-OLOGY
TODAY
2
P
IMMUNOLOGY
TODAY
EP
CP
CY
cf
CCt
3’LCR
The ,x,rt,llels that can be drawn betw~n the @-globin and the ,gH 3wR
suggest mr,ain similti,ies
in lbeir nxcha”isr
of gene Tr-
gulati”“. Haivevw, it is dearly widen, that careful cotidera,i”ns m”s, be taken befare the pm&e mechanisms d LCR funni”” b-2 ““amblg”n”sly
.Sigwd
ca”
N”r cwmp!.?, ree Ref. 60,. .‘uthoygb
many quertions remin “nanswered. I, is h”ped that hrrther study of the IgH LCR will n”, Ml,y reveal ylme Of ,be mecbanisnlr Ip g”ta,ing IgH erpreai”n.bu,mayafs”
pmvidcrhe ~WN
ing the genenclic pmrrrrer ““iqve m this laur,
hrshldy
including CSR and
rnmanc byprm”,a,i”n.
NOVEMBER
1997
IMMUNOLOGY
lODAY
40 Su. L.K. and Kadesch.T. (1990)Mol. , Staudt, L.M. and Len&o, M.J. (1991)Atw~a
Rnv hm~ttnl
9, 373-398
I :I Chm, J.,Tmunstme,M., Att, FW. c’lnl. (1993)lrrl Iwmrrmd5, 647-656 / , Wabl, M.R. and Burrows,P.D tlYR4)Prw NatI
Arnd So
U. S A 81.
!452-2455 1 Cockerill,PN (1990 N~rclc..Arrds Rrr II), 2643-2648 ; Jenuwein,T., Forrester,WC., Fem.mdezherrcm, LA. cl nl. fl997l Nntwc U%5,269-272 i Hemcher, RF., Kaplan,M.H., L&z, D.L cl RI (1995)Cntt~ Da 9,
Crll.
Biol.lt’ 2619-2624
41 Schultz,C.L. and Coffman,R L. (1991)Cwr Opbt.hsawrol.3,350-354 42 DameIs,G.A. and Lie&r, M.R. (1996)Gun. Top.M~cmbiol. fmmmrol. 217, 171-189 43 Jung,S., RaJewsky, K. and Radbmch,A. (1993)5&vce 259,984_9S7 44 Zhang, J., Bottam,A., Li, S., Stewart,V. and Alt, EW. (1993) EMBO/. 12,3529-3537 45 X”, L., Corham, B., Li, SC. etRI. (1993)Pmt. Nntl. Acad. SCI.
I.!. S. A. 90,
3705-3709
1
46 Arulampalam. V., Grant, P.,Paellinger,L. and Pettersson, S. (1995) Mol. Itrm”“rol32, 1369-1375 47 Busslinger,M. and Urbanek,t?(1995)CUR.Opin.Gnat. Dm. 5,595-601
IO Eckhardt,L.A. and Bimhtein,B.K. (19X5)Mol. Cell.Eiol.5,856-86E
321-330
!Q67-3062 ’ *7 Steger,D J,and Workman,J.L.(1996)Ewwys lg. 875-%4 Scheuemwm,R.H. and Chen, U. (1989)Gtws Den 3,1255-1266 j; ) Barbwas,A., Super&Fur@,C. and 6ussltnger.M. (1987)Cell50,347-359
48 Sba,WC., Lieu, H.C., Tuomanen,E.I. and Baltimore.D. (1995) CellSO, al. W%)
/. Immtmd.
I1 Neuberger,MS. and Calabi, F. (1983)Natun 305,240-243 I2 Pettersson, S., Cook, G.P.,Bruggemann,M., Williams,C.T. and
49 Snapper,C.M., Zelamwski, I?, Row, RR. rl la-191
Ueuberger, MS. (1990)Nnfw’ 344,165-168
50 Michaclsmt,J.S.,Singh, M., Snapper,CM. P, al. (19%) J. fmmusd.156,
I3 Lieberson,R., Cimtnini,S.L., Birshtein,B.K.and Eckhardt,L.A. (1991) Vuclcic AcrdsRcs.19.933-937
2826-2839
156,
I4 Dartavach,t?,Williams,C.T., Campbell,K., Pettersso”,S. and
51 Lindersw”. Y., Newath, M., Cmsr, D. and P&te”mon,S. (1997) Eur.J. Immusd.27,468-475
Ueuberger, MS. (1991)Eur.J.bswunol.21.14994504
52 Elenich,LA., Ford, C.S.and Dunnick, W.A. (1996)J. fmmunol.157,
I5 Matthias,P.and Baltimore,D. (1993)Mol. Cell.Biol.13, 1547-1553
176-182
I6 Madison,L. and Groudi”e8M. (1994)Gctrcs Drrr 8.2212-2226
53 Towns, T. and Behrtnger,R. f1990)TwmfsGovt. 6,219-223
17 Gi”nnini, S.L., Singh,M., Calv”, C.E. Ding, C. and Birshtein,B.K. 1993)/. Int1ttamd. 150, 177%17&l I8 Chauveau,C. and Copne,M. (1996)Nat. Gcad. 14.15-16 I9 Mills, EC., Hartndranath,N., Mitchell,M. and Max, E.E. /. Exp. Med. ,in press) I Chen,C. and Birshtein,6.K. (1997)/. Irtmmwml. 159,1310-1318
56 Milot, E.. Stmuboulis,J.,Trimborn,T. d al. (1996)Ccl1S7,lMll4 57 Fiering,S., Epncr.E., RobiMon, K. cf d. (1995)Cmn &v. 9,22C3-2213 51) Hug. B.A., WewJschmklt,R.L., Fierf”g,S. ef al. (1996)Mol. Ctfl. Bfd. 16,25l%2912
ll Cmnt, P.A.,Arulampalam,V., Ahrlund-Richter,L. and Pettersson, S. 1992)NucfcicAcidsRes.20,4401-4406 t.2 Grant, P.,Thompson,C.B.and Pettenscn,S. (1995)EM60 J.14.45014513 23 Atulampalam.V.,Grant, P.A.,bmuelsson,A., Lendahl,U. and Petteram, S. (1994)Eur.J.Itnma,tof.24, 1671-1677 U Grant, P.A.,Andersson,T., New&h, M.F. t’, nl. (1996)EMsO/. 15, 5691-6700 25 Cogne,M., Lansford,R., Bottam,A. etnl. (1994)Cell77,737-747 26 Lieberson,R., Ong, J.,Shi, X. and Eckhadt, L.A. (1995) EMftO/. 14, 6229-623a 27 Gregor,P.D. and Morrison,S.L. (19%) Mol. Cell.Biol.6, 19Ll3-1916 26 Arulampalam.V., Furebring.C., hmuelsson, A. ul RI.(1996) lrtt. Imslll#tol8, 1149-1157 29 Jenuwein,T. and Gmwhedl, R. (1991)Gws Dru. 5,932-943 30 Corcoran,L.M., Karvelas,M., Nossal,G.J.V.etRI. (1993)Got‘s Dnr 7, 57~582 31 Luo. Y.. Fujii. H.. Center, T. and Rwder, R.C. (1992)Cell71.231-241 32 Luo. Y. and Roeder,RG. (199%Mol. Cell.Biof.15.41154124 33 Gstaiger,M.. Cargiev, 0.. van Leeuwen,H., van der Vliet, t?and Schaffner,W. (19%) EMS0 J. 15,2781-2790 34 Strubin.M.. Newell,J.W.and Matthias,f! (1995)Cvll80,497-506 35 Kim, U., Qtn, X.F.,Gong,S. ct nl. (19961N~brre383, 542-547 36 Schubart,D.B.. Rolink,A., Kouo-Vilbois,M.H., Bottert,E and Matthias,P.(1996)Nfllun 383.53%542 37 Zwllling, S.. Annweiler,A. and Wirth, T. (1994) NaclncActdrRq22, 1655-1662 M PftsterwP., Annweiler,A., Ullmer,C.,Corcoran,L.M. a”4 Wirlh. T. (1994)EMBOJ. 13.1655-1663 39 petersson,I?.Grant. P.,Andersso”,P. rf nl. (1995)Jnunnsologisf 3, 146-151
NOVEMBER
I997
54 Wiigerde,M., Gnnveld. E and Fraser,f?(1995)Nalu1?377,209-213 55 Ellis,J..Tan-Un, K.C., Harper,A. rt al. (19%) EMBO J. 15,56%56S
59 Ofron. E.N., Arnold, H.H., Rtgbp P.W.and W&i, Il.). (1996)CcffS5,14 60 Martin, D.L. Fiering,S. and Gmudine,M. (1996)Cwr. O@. Gene&Dcv. 6,488495