Evolution of alpha q- and beta-tubulin genes as inferred by the nucleotide sequences of sea urchin cDNA clones

Abstract

Evolutionary studies on the tubulin multigene families were initiated by nucleotide sequence analysis of cDNA clones complementary to sea urchin (Lytechinus pictus) testis alpha- and beta-tubulin cDNA clones (p beta 1, p beta 2, p beta e) demonstrated the existence of tubulin mRNA heterogeneity. p beta 2 and p beta 3 contain identical tubulin-coding regions and extremely similar 3' untranslated sequences, including a polyadenylation signal (AAUAAA). However, p beta 2 contains an additional region of 3' untranslated sequence which includes a second polyadenylation signal. These two sequences may be allelic, representing products of alternative transcription termination or processing pathways. p beta 1 and p beta 2 (or p beta 3) cDNAs almost certainly correspond to transcripts of distinct but evolutionarily related genes. Examination of the available coding portions showed that they differ only by a few silent nucleotide substitutions and the deletion/insertion of one codon; most of the differences are clustered within the last 15 3'-end codons. In contrast, their 3' untranslated sequences are considerably divergent. Nucleotide alignment in this region was feasible by considering specific point and segmental mutations, mainly T in equilibrium or formed from C transitions and small deletions/insertions associated with small direct repeats. The sea urchin alpha- and beta-tubulin cDNA and corresponding protein sequences were compared with previously described tubulin cDNA and protein sequences from other organisms. Both alpha and beta tubulins are very conserved proteins, evolving with a rate comparable to that of histones. Analysis of the nucleotide divergence of the coding cDNA regions showed that replacement sites have changed with a rate 20-175 times lower than that of the silent sites. Among the 177 codons compared between the sea urchin testis and chick brain beta-tubulin cDNAs, there are 7 conservative amino acid replacements and the deletion/insertion of two codons. Most of these changes are clustered near the C-terminus. The 161-amino acid portion of chick brain, rat and porcine alpha-tubulin sequences differs by 3 conservative amino acid replacements from the corresponding sea urchin testis alpha-tubulin sequence. The compared interspecies 3' untranslated sequences are very divergent.