Date of Award

Winter 1999

Project Type

Dissertation

Program or Major

Biochemistry

Degree Name

Doctor of Philosophy

First Advisor

John Collins

Abstract

The mut-2 mutator plays multiple regulatory roles in the germ line of C. elegans. In addition to regulating germ line transposition of at least four distinct transposon families (Tc1, Tc3, Tc4 and Tc5) mut-2 is implicated in chromosome segregation. Animals that harbor the mut-2 mutator produce broods with a higher incidence of males phenotype (Him) as a result of an increase in X chromosome non-disjunction during meiosis.

Using the Him phenotype conferred by mut-2(r459), I mapped the gene to the dpy-14 sem-4 interval on LGI. However, efforts to identify a molecular clone of the gene were hampered because the available phenotypes were unsuitable for standard transformation rescue approaches. This lead me to re-examine the temperature sensitive behavior of mut-2(r459); the original mut-2 isolate, TR674, is sterile at 25°C. Previously it was assumed that the ts behavior was related to its heritage. TR674 is derived from a hybrid of two common laboratory strains, N2 and Bergerac (BO). N2 is fertile at 25°C but BO is sterile. Analysis of the immediate ancestors and recombinants derived from mut-2-bearing strains demonstrated that the ts sterile phenotype cosegregates with mut-2 and is unrelated to its BO ancestry. Closer inspection revealed that sterility results from a defect in gametogenesis. The gonads of animals raised at 25°C have few, morphologically abnormal sperm. The oocytes appear to be normal but attempts to cross-fertilize with wild type sperm at 25°C failed to produce any viable progeny.

This thesis describes how I took advantage of the ts phenotype to achieve rescue and identify a candidate gene. I demonstrated rescue and restored fertility with a small, genetically defined free duplication, hDp65. Using standard DNA transformation techniques, I defined the relative physical position of the duplication endpoint within the dpy-14 sem-4 interval thereby reducing the mut-2 interval to a region spanned by two cosmids, C30F12 and H06O01. Of these two, H06O01 rescued the ts sterile phenotype. The sequence represented in this cosmid includes a gene with similarity to the chromo domain-helicase-DNA binding protein, CHD1. This gene is a member of the SWI/SNF superfamily of transcriptional regulators and encodes a protein with several motifs shared by factors that regulate transcription by remodeling chromatin architecture.

CHD1 represents a compelling candidate for the mut-2 gene given that mut-2 plays a role in regulating multiple transposons genome wide and participates in chromosome segregation and gametogenesis. Confirmation awaits rescue by the CHD1 gene and sequencing of the molecular lesion responsible for mut-2(r459).

The work that I have described moves us closer to identifying and cloning the mut-2 gene. Ultimately, the characterization of mut-2's structure and function will elucidate the mechanisms regulating transposition in C. elegans, allow better use of transposons as molecular tools and provide insight into the processes that maintain integrity and plasticity of the genome throughout life.

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