Date of Award

2010

Degree Type

Thesis

Department

Biological, Geological and Environmental Sciences

First Advisor

Doerder, Paul

Subject Headings

Tetrahymena, Antigens -- Analysis, Immunoblotting, Tetrahymena thermophila, Immobilization antigen, SerH, Concerted evolution

Abstract

Tetrahymena thermophila express a major cell surface protein known as the immobilization (i-) antigen which coats the entire cell surface including the cilia. T. thermophila has several different i-antigens which are expressed in a mutually exclusive manner under varying environmental conditions. When the genes for these i-antigens were sequenced it was found that they encode proteins consisting of imperfect repeats with cysteine periodicity. The best characterized of these i-antigens are those specified by alleles at the SerH i-antigen locus. The H proteins all contain a section of 8-cysteine containing imperfect repeats. The presence of repeats allows for the possibility that SerH genes evolve, at least in part, by concerted evolution, a process in which the repeats of the gene evolve together so that the repeats within one sequence would be more similar to each other than they would be to the repeats of any other sequence. A previous study (Mol. Biol. Evol. 23: 608-614) found evidence that SerH genes evolve via a mix of vertical transmission and concerted evolution. This study characterized 20 SerH alleles from wild samples and further explored the mode of evolution of SerH i-antigen. Using bioinformatic tools, SerH alleles were characterized with respect to nucleotide diversity, repeat structure, codon usage, and sequence evolution. The encoded proteins were examined for amino acid composition, cysteine periodicity, and potential secondary structure. A model of the i-antigen structure was presented. Standard bioinformatic tests for evolution provided no evidence that SerH genes are positively selected. Neighbor-joining trees of the 8 cysteine-containing confirm that SerH genes evolve through a mix of primarily vertical transmission and concerted evolution

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