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The Su(var)3-9/eIF2γ locus of Drosophila melanogaster and the advantages and disadvantages of this compound gene arrangement Harrington, Michael John

Abstract

The Su(var)3-9/eIF2γcompound locus of Drosophila melanogaster is comprised of two genes. Alternative splicing produces two transcripts which encode very different proteins. Su(var)3-9 is a putative chromatin protein while eIF2γ is a translation initiation factor. A genetic dissection of this locus has allowed two hypotheses to be tested. The genes in compound loci have a complex arrangement and share regulatory and transcribed regions. The first hypothesis proposed that these loci are therefore more vulnerable to mutations than independent genes. A characteristic of many compound loci is an ancestral or functional relationship between the pair of genes. This formed the basis of the second hypothesis, whether there is a relationship between the function of the Su(var)3-9 and eIF2γ proteins. In summary, this thesis investigated one possible benefit (coordinated regulation of functionally related genes) and one disadvantage (increased susceptibility to mutations) of compound loci. Using Southern blot, DNA sequencing, Northern blot, and phenotypic analyses, mutant alleles were characterized and could be divided into three categories. Those that affected both genes equivalently (322,P25,P17,P17rv12, and P17rv9), those that disrupted only the Su(var)3-9 gene (311,318, and 330), and one that primarily affected the Su(var)3-9 gene (336). These alleles and expression of a transformed eIF2γ gene allowed mutant phenotypes to be associated with each gene. Four independent alleles were due to insertions of transposable elements into the region of the locus common to both genes. Each mutation disrupted two genes and hence two cellular processes. This reveals the sensitivity of this compound locus to mutations and thus supports the first hypothesis. Mutations in the Su(var)3-9 gene cause a dominant suppressor of PEV phenotype and interact with the Histone gene cluster (HIS-C). Mutations in the eIF2γ gene are recessive lethal and possess several mutant phenotypes associated with Minute loci. Most Minute loci encode ribosomal proteins, mutations in this translation initiation factor share their dosage sensitive phenotypes. Because the functions of each gene, as determined by this genetic analysis, were independent, there was no evidence supporting the second hypothesis which examined the relationship between the two genes.

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