Evolution, organization, and expression of alpha-tubulin genes in the Antarctic fish Notothenia coriiceps - Adaptive expansion of a gene family by recent gene duplication, inversion, and divergence

To assess the organization and expression of tubulin genes in ectothermic v ertebrates, we have chosen the Antarctic yellowbelly rockcod, Notothenia co riiceps,as a model system. The genome of N. coriiceps contains similar to 1 5 distinct DNA fragments complementary to alpha-tubulin cDNA probes, which suggests that the alpha-tubulins of this cold-adapted fish are encoded by a substantial multigene family. From an N. coriiceps testicular DNA library, we isolated a 13.8-kilobase pair genomic clone that contains a tightly lin ked cluster of three cr-tubulin genes, designated NeGTb alpha a, NcGTb alph a b, and NcGTb alpha c. Two of these genes, NcGTb alpha a and NcGTb alpha b , are linked in head-to-head (5' to 5') orientation with similar to 500 bp separating their start codons, whereas NcGTb alpha a and NcGTb alpha c are linked tail-to-tail (3' to 3') with similar to 2.5 kilobase pairs between t heir stop codons. The exons, introns, and untranslated regions of the three alpha-tubulin genes are strikingly similar in sequence, and the intergenic region between the alpha a and ab genes is significantly palindromic. Thus , this cluster probably evolved by duplication, inversion, and divergence o f a common ancestral alpha-tubulin gene. Expression of the NcGTb alpha c ge ne is cosmopolitan, with its mRNA most abundant in hematopoietic, neural, a nd testicular tissues, whereas NcGTb alpha a and NcGTb alpha b transcripts accumulate primarily in brain. The differential expression of the three gen es is consistent with distinct suites of putative promoter and enhancer ele ments. We propose that cold adaptation of the microtubule system of Antarct ic fishes is based in part on expansion of the alpha- and beta-tubulin gene families to ensure efficient synthesis of tubulin polypeptides.