Volvox carteri α2- and β2-tubulin-encoding genes: regulatory signals and transcription

Abstract

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 αtub and βtub genes with their Cr counterparts. The Vc genome contains two αtub and two βtub genes. We report here the sequences of the α2tub and β2tub genes, and thus complete the set of four tub sequences. The two αtub and two βtub genes code for identical 451 (α) and 443 (β) amino acid (aa) polypeptides; they differ from the Cr homologs in two (α) and one (β) 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 α2tub and β2tub starts with an A, 26 by (α 2) or 25 by (β 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 αtub. A conserved TGTAA downstream from the translation stop codon represents the algal poly(A)-addition signal (in both Vc and Cr). Northern analyses and reverse transcription (RT) followed by polymerase chain reaction have demonstrated that all four tub RNAs are present at all stages of the Vc life cycle