REGULATION AND EVOLUTION OF THE SINGLE ALPHA-TUBULIN GENE OF THE CILIATE TETRAHYMENA-THERMOPHILA

The single alpha-tubulin gene of Tetrahymena thermophila was isolated from a genomic library and shown to encode a single protein. Compariso ns of the rates of evolution of this gene with other alpha-tubulin seq uences revealed that it belongs to a group of more evolutionarily cons trained alpha-tubulin proteins in animals, plants, and protozoans vers us the group of more rapidly evolving fungal and variant animal alpha- tubulins. The single alpha-tubulin of Tetrahymena must be used in a va riety of microtubule structures, and we suggest that equivalently cons erved alpha-tubulins in other organisms are evolutionarily constrained because they, too, are multifunctional. Reduced constraints on fungal tubulins are consistent with their simpler microtubule systems. The a nimal Variant alpha-tubulins may also have diverged because of fewer f unctional requirements or they could be examples of specialized tubuli ns. To analyze the role of tubulin gene expression in regulation of th e complex microtubule system of Tetrahymena, alpha-tubulin mRNA amount s were examined in a number of cell states. Message levels increased i n growing versus starved cells and also during early stages of conjuga tion. These changes were correlated with increases in transcription ra tes. Additionally, alpha-tubulin mRNA levels oscillate in a cell cycle dependent fashion caused by changes in both transcription and decay r ates. Therefore, as in other organisms, Tetrahymena adjusts alpha-tubu lin message amounts via message decay. However the complex control of alpha-tubulin mRNA during the Tetrahymena life cycle involves regulati on of both decay and transcription rates. (C) 1994 Wiley-Liss, Inc.