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)

Reverse transcriptase families and a copia-like retrotransposon, Osser, in the green alga Volvox carteri.

By using the polymerase chain reaction (PCR) we have isolated and sequenced two distinct families of reverse transcriptase (RT) sequences from the genome of the colonial alga, Volvox carteri. Probing a genomic library with these RT clones revealed copia-like retrotransposons. One of these elements, named Osser, is 4,875 bp long, bordered by 197-bp identical long terminal repeats (LTRs), and shows the typical organization of retrotransposons belonging to the copia-Tyl group. This is the first complete copia-like retrotransposon sequence described in a green alga.

Organization and transcription of Volvox histone-encoding genes: similarities between algal and animal genes.

The nucleotide sequences of two non-allelic histone H2A-H2B gene loci of the green alga Volvox carteri have been determined. Each locus contains a divergently arranged H2A-H2B gene pair. The encoded proteins differ in one (H2A) and 16 positions (H2B), respectively. The coding regions are separated by short intercistronic segments (256 bp and 298 bp) containing TATA boxes and a central tandem repeat of a conserved 20-bp element as the putative histone-specific transcription signals. The 3'-untranslated regions exhibit a characteristic 3'-palindrome and weakly conserved spacer elements. Transcription in one gene locus was shown to initiate 48 bp upstream from H2A and 59 bp upstream from H2B. Contrary to higher plants, V. carteri histone mRNAs are nonpolyadenylated. S1 mapping and Northern-blotting experiments indicated that V. carteri histone mRNAs are terminated at the 3'-palindrome by the same mechanism that operates in vertebrates and sea urchins.

Flow cytometer study of anterior-like cells in Dictyostelium discoideum.

The Dictyostelium discoideum asexual fruiting body consists of spores, stalk and basal disk cells. Recently, a fourth cell class has been proposed. It has been suggested that these cells originate from anterior-like cells that remain undifferentiated. Anterior-like cells are randomly distributed among prespore cells in the posterior part of the slug. Here monoclonal antibodies that recognize the surface of prespore cells (MUD1), and spores (MUD3) are used in a quantitative flow cytometer assay to demonstrate that this fourth cell class does not exist in the mature fruiting body. However, the tip cells are slow to differentiate, and hence immature fruiting bodies contain a small population of undifferentiated tip cells. We confirm that anterior-like cells represent a large percentage of the non-prespore cell population in the slug. In this report we were unable to distinguish these anterior-like cells from prestalk cells on the basis of size or monoclonal antibody staining.

Transitions in Dictyostelium discoideum behaviour: influence of calcium and fluoride on slug phototaxis and thermotaxis.

Phototaxis and thermotaxis by slugs of Dictyostelium discoideum show transitions that result in bimodality in phototaxis and temperature-dependent orientation up or down temperature gradients. New steps in the sensory transduction chain for these kinds of behaviour are elucidated from studies using inorganic salts (Ca2+, EGTA, KF) and several mutants. KF enhances bimodality of phototaxis, improves the accuracy of thermotaxis, and affects the transition temperatures from positive to negative thermotaxis. Changing the Ca2+ concentration has effects on both phototaxis and thermotaxis. At low Ca2+ concentrations phototaxis and thermotaxis are enhanced. In the presence of EGTA or high concentrations of Ca2+ phototaxis becomes bimodal, thermotaxis is impaired and spontaneous turning is suppressed. These results obtained by changing Ca2+ concentrations are analogous to those obtained previously with sensory transduction mutations, which coordinately affect phototaxis and thermotaxis.