VOLVOX-CARTERI ALPHA-2-TUBULIN-ENCODING AND BETA-2-TUBULIN-ENCODING GENES - REGULATORY SIGNALS AND TRANSCRIPTION

Microtubules (MT) carry out several specialized morphogenetic function s in the multicellular green alga Volvox carteri (Vc), in addition to functions also executed in its closest unicellular relative, Chlamydom onas reinhardtii (Cr). To find out if these differences in morphogenet ic complexity are reflected in tubulin (Tub) differences, we have comp ared the Vc alpha tub and beta tub genes with their Cr counterparts. T he 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 com plete the set of four tub sequences. The two alpha tub and two beta tu b 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 betwee n sibling genes are much more frequent in Vc than in Cr (12 vs. 2%), p robably owing to a more stringent codon bias in the latter alga. Trans cription 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 elem ent upstream and a G + C-rich sequence downstream from the TATA box ar e conserved in all tub of both species. Moreover, a 28-bp element of c onserved 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 ). Northern analyses and reverse transcription (RT) followed by polyme rase chain reaction have demonstrated that all four tub RNAs are prese nt at all stages of the Vc life cycle.