Insulinoma

I n s u li n o m a 1037 other IS elements (e.g., one subgroup of the IS5 family). Other control mechanisms may occur at translation termination. In some cases, the translation termination codon of Tpase genes is located within their IRR sequences, while in others the transposase gene simply does not possess a termination codon. Among the latter cases, the IS is known to insert into a specific target sequence in which the target direct repeat produced on insertion itself generates the Tpase termination codon. This has been observed for certain members of the IS630 family. The significance of these arrangements may be to couple translation termination, transposase binding, and transposition activity. Early studies of IS1 and IS50 demonstrated that impinging transcription from outside reduces transposition activity. Transcription may disrupt the formation of the transposition complexes known as transpososomes in which transposase and the transposon ends are intimately bound. Tpase stability can also contribute to control of transposition since it can limit activity both temporally and spatially. This may explain the observation that several Tpases function preferentially in cis (see below). Derivatives of the IS903 Tpase that are more resistant to the E. coli Lon protease than the wild type protein are more active and exhibit an increased capacity to function in trans (see below). Early studies indicated that transposition activity of some elements was more efficient if the transposase is provided by the element itself or by a transposase gene located close by on the same DNA molecule. This preferential activity in cis reduces the probability that transposase expression from a given element will activate transposition of related copies elsewhere in the genome. The effect can be of several orders of magnitude. It presumably reflects a facility of the cognate transposases to bind to transposon ends close to their point of synthesis and is likely to be the product of several phenomena such as expression levels and protein stability. Another contributing factor may derive from the domain structure of known transposases (see above) in which the DNA binding domain is located in the N-terminal end of the protein. This arrangement would permit preferential binding of nascent transposase polypeptides to neighboring binding sites. Indeed, the N-terminal portion of several Tpases exhibits a higher affinity for the ends than does the entire transposase molecule, suggesting that the C-terminal end may mask the DNA binding activity of the N-terminal portion.

Further Reading

Berg DE and Howe MM (eds) (1989) Mobile DNA. Washington, DC: American Society for Microbiology Press.

Chaconas G, Lavoie BD and Watson MA (1996) DNA transposition: jumping gene machine, some assembly required. Current Biology 6: 817±820. Haren L, Ton-Hoang B and Chandler M (1999) Integrating DNA: transposases and retroviral integrases. Annual Review of Microbiology 53: 245±281. Mahillon J and Chandler M (1998) Insertion sequences. Microbiology and Molecular Biology Reviews 62: 725±774. Mizuuchi K (1992) Transpositional recombination: mechanistic insights from studies of Mu and other elements. Annual Review of Biochemistry 61: 1011±1051. Mizuuchi K (1997) Polynucleotidyl transfer reactions in sitespecific DNA recombination. Genes to Cells 2: 1±12. Rice P, Craigie R and Davies DR (1996) Retroviral integrases and their cousins. Current Opinion in Structural Biology 6: 76 ±83. Saedler H and Gierl A (eds) (1996) Transposable Elements, Current Topics in Microbiology and Immunology, Vol. 204. Berlin: Springer-Verlag.

See also: Escherichia coli; Transposable Elements

Insertion, Insertional Mutagenesis See: Chromosome Aberrations; DNA Cloning; In vitro Mutagenesis; Mutation

Insulinoma C S Grant Copyright ß 2001 Academic Press doi: 10.1006/rwgn.2001.1584

Insulinoma occurs primarily in one of two principal forms, sporadic and familial, specifically as one component of the multiple endocrine neoplasia type 1 (MEN-1). MEN-1 is a clinical syndrome inherited in an autosomal dominant pattern, and includes primary hyperparathyroidism, multiple duodenopancreatic endocrine tumors (of which insulinoma is one type), and pituitary adenomas. No specific genetic abnormality has been consistently identified as the cause of sporadic insulinomas, whereas the recently cloned gene responsible for inheritance of MEN-1 has been mapped to chromosome 11q13. This gene contains 10 exons that encode a 610-amino acid protein product, menin. Research suggests that the MEN-1 gene is a tumor suppressor gene.

Further Reading

Chandrasekharappa SC, Guru SC, Manickam P et al. (1997) Positional cloning of the gene for multiple endocrine neoplasia-type 1. Science 276: 404.

1038

Integrase

Larsson C, Skogseid B, Oberg K, Nakamura Yand Nordenskjold M (1988) Multiple endocrine neoplasia type 1 gene maps to chromosome 11 and is lost in insulinoma. Nature 332: 85±87.

See also: Adenoma; Multiple Endocrine Neoplasia

Integrase N Grindley Copyright ß 2001 Academic Press doi: 10.1006/rwgn.2001.0698

The term integrase is used to describe the following two enzymes: 1. An enzyme (Int) responsible for catalyzing the breakage and rejoining of DNA during the insertion of a bacteriophage genome into (and its excision from) its chromosomal attachment site by the process of site-specific recombinases. Most Int proteins belong to the tyrosine recombinase family of site-specific recombinases but several examples of serine recombinases are also known. 2. An enzyme (IN) encoded by retroviruses that is responsible for the 30 processing of retroviral DNA and insertion of the processed DNA into a genomic target. IN proteins are derived by proteolysis from the C-terminus of the gag-pol polyprotein, and belong to the DD(35)E family of transposases. See also: Integrase Family of Site-Specific Recombinases; Phage l Integration and Excision; Retroviruses; Site-Specific Recombination; Transposable Elements

Integrase Family of SiteSpecific Recombinases A Landy Copyright ß 2001 Academic Press doi: 10.1006/rwgn.2001.1452

The Int family of recombinases belongs to the general class of proteins that act on specific DNA sequences to effect deletion, insertion, or inversion of large segments of genomic DNA. The approximately 100 known proteins in this family are found in archaebacteria, eubacteria, and eukaryotes. Sometimes referred to as the tyrosine integrase family, they are distinguished by their use of a tyrosine nucleophile and five

highly conserved basic residues to catalyze DNA cleavage and ligation reactions in the absence of highenergy cofactors. Another hallmark for the recombinases in this family is a sequential strand exchange mechanism that generates a four-way DNA junction (Holliday junction) as a recombination intermediate. The biological roles of various Int family members include copy number control and stable inheritance of circular replicons, the integration and excision of viral chromosomes into and out of the chromosomes of their respective hosts, the regulation of expression of cell surface proteins, conjugative transposition, the movement of antibiotic resistance genes into and out of transposable elements and plasmids, and the relaxation of positive and negative supercoils during eukaryotic DNA replication repair, recombination, and transcription.

The Reaction The minimal Int family target on DNA consists of a single binding site for a topoisomerase monomer. DNA strand cleavage involves activation of the scissile phosphate by the highly conserved pentad of active site residues (Arg, Lys, His, Arg, His) and formation of a nick with a 50 OH and 30 -phosphotyrosine linkage to the recombinase. This transient covalent intermediate releases one superhelical turn, via mechanics that are not completely understood, and the nick is resealed by a simple reversal of the cleavage step. In the case of Int family-mediated recombination, the minimal DNA target consists of two recombinase binding sites that are positioned as inverted repeats separated by 6±8 bp (called the `overlap region'). Synapsis and proper alignment of two such recombination partners generates a tetrameric complex in which each recombinase protomer carries out the cleavage and ligation of one DNA strand, executed as two sequential pairs of cleavage/ligation reactions. In the first pair of reactions one strand in each partner DNA helix is cleaved and the first three or four bases of the free 50 hydroxyl-terminated strands of the overlap region are swapped and then ligated. This forms a Holliday junction with four continuous DNA strands (see Figure 1). After some rearrangements within the Holliday junction, the intermediate is `resolved' by a reciprocal strand swapping of the second pair of strands so that all four DNA strands have new junctions and two recombinant DNA helices have been generated. The formation of Holliday juction intermediates distinguishes the Int family from the resolvase/invertase family of site-specific recombinases, which use a serine nucleophile to carry out a pair of concerted (rather than sequential) strand exchanges, and from the transposase family of