Volvox carteri alpha 2- and beta 2-tubulin-encoding genes: regulatory signals and transcription.

Microtubules (MT) carry out several specialized morphogenetic functions in the multicellular green alga Volvox carteri (Vc), in addition to functions also executed in its closest unicellular relative, Chlamydomonas reinhardtii (Cr). To find out if these differences in morphogenetic complexity are reflected in tubulin (Tub) differences, we have compared the Vc alpha tub and beta tub genes with their Cr counterparts. The Vc genome contains two alpha tub and two beta tub genes. We report here the sequences of the alpha 2tub and beta 2tub genes, and thus complete the set of four tub sequences. The two alpha tub and two beta tub genes code for identical 451 (alpha) and 443 (beta) amino acid (aa) polypeptides; they differ from the Cr homologs in two (alpha) and one (beta) residues, respectively. Silent nucleotide (nt) exchanges between sibling genes are much more frequent in Vc than in Cr (12 vs. 2%), probably owing to a more stringent codon bias in the latter alga. Transcription of alpha 2tub and beta 2tub starts with an A, 26 bp (alpha 2) or 25 bp (beta 2) downstream from the TATA box. A 16-bp promoter element upstream and a G + C-rich sequence downstream from the TATA box are conserved in all tub of both species. Moreover, a 28-bp element of conserved sequence, and hence of possible functional significance, was found at similar locations in the 5' untranslated region (UTR) of all four alpha tub. A conserved TGTAA downstream from the translation stop codon represents the algal poly(A)-addition signal (in both Vc and Cr).(ABSTRACT TRUNCATED AT 250 WORDS)