Evolution of αq- and β-tubulin genes as inferred by the nucleotide sequences of sea urichin cDNA clones

Evolutionary studies on the tubulin multigene families were initiated by nucleotide sequence analysis of cDNA clones complementary to sea urchin (Lytechinus pictus) tesits α- and β-tubulin mRNAS. Sequence comparisons of three partial β-tubulin cDNA clones (pβ1, pβ2, pβ3) demonstrated the existence of tubulin mRNA heterogeneity. pβ2 and pβ3 contain identical tubulin-coding regions and extremely similar 3′ untranslated sequences, including a polyadenylation signal (AAUAAA). However, pβ2 contains an additional region of 3′ untranslated sequence which includes a second plyadenylation signal. These two sequences may be allelic, representing products of alternative transcription termination or processing pathways. pβ1 and pβ2 (or pβ3) 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 contrst, their 3′ untranslated sequences are considerably divergent. Nucleotide alignment in this region was feasible by considering specific point and segmental mutations, mainly T↔C transitions and small deletions/insetions associated with small direct repeats. The sea urchin α- and β-tubulin cDNA and corresponding protein sequences were compared with previously described tubulin cDNA and protein sequences from other organisms. Both α and β tubulins are very conserved proteins, evolving with a rete comparable to that of histones. Analysis of the nucleotide divergence of the coding cDNA regions showed that relacement sites have changed with a rate 20-175 times lower than that of the silent sites. Among the 177 codons compared between the sea urchinb testic and chick brain β-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 α-tubulin sequences differs by 3 conservative amino acid replacements from the corresponding sea urchin testis α-tubulin sequence. The compared interspecies 3′ untranslated sequences are very divergent.