Evolution, organization, and expression of α-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 vertebrates, we have chosen the Antarctic yellowbelly rockcod, Notothenia coriiceps,as a model system. The genome of N. coriiceps contains similar to 15 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 linked cluster of three cr-tubulin genes, designated NeGTb alpha a, NcGTb alpha 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 their 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 of a common ancestral alpha-tubulin gene. Expression of the NcGTb alpha c gene is cosmopolitan, with its mRNA most abundant in hematopoietic, neural, and testicular tissues, whereas NcGTb alpha a and NcGTb alpha b transcripts accumulate primarily in brain. The differential expression of the three genes is consistent with distinct suites of putative promoter and enhancer elements. We propose that cold adaptation of the microtubule system of Antarctic fishes is based in part on expansion of the alpha- and beta-tubulin gene families to ensure efficient synthesis of tubulin polypeptides.