Genetic control of flower shape in Antirrhinum majus

The asymmetric shape of the Antirrhinum corolla is determined by genes acting differentially along the dorsoventral axis of the flower. Genes so far identified determine asymmetry by acting in dorsal regions. We describe a gene, divaricata, which in contrast to previously identified genes acts in ventral regions of the flower. We show by the analysis of mutant combinations that the divaricata gene is negatively regulated by the dorsal genes cycloidea and dichotoma. In addition, we show by the analysis of chromosomal duplications that the divaricata gene acts in a dosage-dependent manner, suggesting that the level of its product is critical for determining ventral identities.

Heat-shock-induced protein synthesis is responsible for the switch-off of hsp70 transcription in Tetrahymena

We had previously described that new RNA synthesis is required for expression of the heat shock protein HSP70. Here, we find that the HSP70 mRNA decreases its levels under stress conditions, heat shock (HS) or arsenite (As), and that its levels start to decline at the same time as maximal HSPs synthesis (including HSP70) occurs. This suggests that regulation of the hsp70 gene is mainly exerted at the transcriptional level. Accumulation of the HSP70 mRNA in cells stressed in presence of cycloheximide (CHX), indicates that (a) protein(s) non-existent before stress, possibly HSP70 itself (which is shown here to be relatively stable), is involved in negatively regulating hsp70 expression. Since degradation of the HSP70 mRNA is also shown to occur in cells heat-shocked under CHX, as seen from decay of its levels upon addition of actinomycin D (AMD), the protein(s) must repress hsp70 expression at the transcriptional level. Other conditions that affect normal protein synthesis, namely the translation inhibitor puromycin and the arginine-analog canavanine (shown here to be stress inducers in Tetrahymena pyriformis), also cause a delay in transcription-arrest of the HSP70 mRNA. Under severe stress conditions of HS (36 degrees C) or As (350 microM), the levels of HSP70 mRNA are higher than under mild stress conditions, however, no significant difference is seen in the pattern of HSP70 mRNA decay.

The mechanisms of tubulin messenger regulation during Tetrahymena pyriformis reciliation

The reciliation of Tetrahymena pyriformis cells is accompanied in the first minutes by a transient induction of stress mRNAs, i.e. hsp70 and ubiquitin. At the same time an accentuated and coordinate reduction in the amount of alpha- and beta-tubulin mRNAs is observed as analyzed by Northern blot hybridization using the homologous genomic tubulin probes. Between 60 and 120 min after the onset of reciliation, tubulin transcripts in the cytoplasm reach higher values than in exponentially growing cells. Run-on transcription assays reveal that the decrease in tubulin mRNA levels is not caused by a decrease in transcription of tubulin genes. The results obtained show that the apparent tubulin gene transcription rate is increased in reciliating cells from 15 min up to 90 min. The block of transcription using actinomycin D shows that hsps are not implicated in the destabilization of tubulin mRNA during the first minutes of reciliation. The effects of the inhibitors of protein synthesis, cycloheximide and pactamycin, on tubulin mRNA levels suggest that the translational apparatus plays a role in the stability of tubulin mRNA in cells reciliating for 15 and 30 min. Experiments using the microtubule-polymerizing drug taxol also show that tubulin mRNA destabilization is not a simple consequence of a temporary increase in free tubulin subunit pools resulting from cilia resorption.

Known heat-shock proteins are not responsible for stress-induced rapid degradation of ribosomal protein mRNAs in yeast

We have previously shown that the heat-induced enhanced decay of yeast mRNAs encoding ribosomal proteins (rp-mRNAs) requires ongoing transcription during the heat treatment [Herruer et al. (1988) Nucl. Acids Res. 16, 7917]. In order to determine whether this requirement reflects the need for heat-shock protein (hsp), we analysed the effect of heat shock on rp-mRNA levels in several yeast strains in which each of the heat-shock genes encoding hsp26, hsp35 or hsp83 had been individually disrupted. In all three strains we still observed increased degradation of rp-mRNAs immediately after the temperature shift, demonstrating that hsp26, hsp35 and hsp83 are not required for this effect. Accelerated turnover of rp-mRNA was also found to occur upon raising the growth temperature of a mutant strain that contains a disruption of the gene specifying the heat-shock transcription factor and in wild-type yeast cells treated with canavanine, an arginine analogue that will be incorporated into all known hsps and that is known to cause misfolding of the polypeptide chain. Latter observation suggests that enhanced rp-mRNA decay is a more general stress-related phenomenon. Taken together, these data strongly indicate that the trans-acting factor required for the increase in the rate of degradation of rp-mRNAs upon stress is not one of the known yeast hsps.

Multiple alpha-tubulin isoforms in cilia and cytoskeleton of Tetrahymena pyriformis generated by post-translational modifications. Studies during reciliation

alpha-Tubulin microheterogeneity was studied in Tetrahymena pyriformis. Using two-dimensional electrophoretic analysis, we found between five and seven alpha-tubulin and four beta-tubulin isoforms in cilia and four or five alpha-tubulins and two beta-tubulins in cytoskeleton. Immunoblotting assay with anti-(acetyl alpha-tubulin) monoclonal antibody 6-11B-1 and [3H]acetate labelling revealed that the alpha-tubulin isoforms are post-translationally modified by acetylation. Our results also show that tubulins in the soluble cytoplasmic fraction are not acetylated. Nevertheless, a slight reaction with the antibody 6-11 B-1 can be observed in the taxol and vinblastine-treated cytoplasmic pool. Pulse/chase experiments using [35S]methionine during cell reciliation have demonstrated that the basic alpha-tubulin isoforms are converted into acidic isoforms in the absence of protein synthesis, suggesting that the basic alpha-tubulin is the precursor of the acidic forms which are found in cilia and cytoskeleton. In-vivo-translation selection demonstrated the existence of a single precursor molecule which corresponded to the most basic alpha-tubulin. Taken together, our results provide evidence for the existence of post-translational modifications, namely acetylation. Nevertheless, other post-translational mechanisms involved in the biosynthesis of microtubules of cilia and cytoskeleton are required to explain the whole alpha-tubulin heterogeneity.

Ubiquitin genes in Tetrahymena pyriformis and their expression during heat shock

The genome of Tetrahymena pyriformis was shown to contain a ubiquitin multigene family consisting of at least four polyubiquitin genes. Three genomic clones with different ubiquitin-coding sequences, were isolated and partially characterized. The complete nucleotide sequence of one of these clones (pTU2) was determined and showed two open reading frames (ORFs) at opposite ends of the cloned DNA insert. A comparison of the predicted amino acid (aa) sequence of T. pyriformis ubiquitin-coding unit with those from other organisms indicated a high degree of homology. However, Tetrahymena ubiquitin contained two aa substitutions at positions 16 (Asp) and 19 (Ala). Interestingly, the first pTU2 ORF showed two extra triplets coding for Ser and Gln, upstream from TGA. This feature is different from all the polyubiquitin genes thus far sequenced. Regions flanking the 3' and 5' ubiquitin-coding sequences presented several conserved motifs. The 5' flanking sequence of the second ORF of pTU2 contained one heat-shock element. We therefore studied the expression of the ubiquitin genes under stress conditions. The results showed that they are heat-inducible and that a new specific 1.6-kb mRNA appeared. These results suggest that the regulation of ubiquitin genes is important in T. pyriformis under thermal stress conditions.

Destabilization of tubulin mRNA during heat shock in Tetrahymena pyriformis

The regulation of tubulin gene expression was studied in Tetrahymena pyriformis cells during heat shock (shift from 28 degrees C to 34 degrees C). Fluorograms of two-dimensional gels of radiolabelled proteins synthesized during thermal stress revealed that tubulin synthesis is highly repressed when compared with that of exponentially growing cells. The variation in the levels of alpha and beta-tubulin mRNAs was analyzed by Northern-blot hybridization using homologous genomic probes (alpha TT and beta TT1). The results obtained show that heat shock induces a drastic and coordinate reduction in the amount of alpha and beta-tubulin mRNAs isolated from polysomes. This decrease is not due to a shift from the polysomes to the post-polysomal fraction because it was also observed when total cytoplasmic mRNAs were analyzed. Run-on transcription experiments were performed in order to examine whether repression of transcription in heat-shocked cells could explain that reduction. The results obtained show that the apparent rates of tubulin gene transcription are not significantly modified, but on the contrary increase slightly in cells heat-shocked for 15 min and 30 min. The effects of inhibitors of protein synthesis, cycloheximide and pactamycin, on the destabilization of tubulin mRNAs were tested in heat-shocked Tetrahymena cells. Our results revealed that in the presence of these inhibitors, tubulin mRNAs become more stable thus suggesting that an induced factor may be involved in the degradation of alpha and beta-tubulin mRNAs during heat shock.

Stress response of Tetrahymena pyriformis to arsenite and heat shock: differences and similarities

Molecular and cellular events associated with the response of Tetrahymena pyriformis to stress induced by sodium meta-arsenite, have been examined by pulse-labelling experiments. This stress agent induces the synthesis of two main groups of proteins, with molecular masses in the ranges 70-75 kDa and 25-29 kDa, together with other proteins of molecular masses 92, 83, 46, 42 (two species), 36 and 35 kDa. Comparison of the results with those of a previous study concerning the response of T. pyriformis to heat-shock, shows that the two main groups of proteins, as well as the 92-kDa and 35-kDa species, which seem to be similarly induced by both types of stress, display similar or identical peptide maps. Other stress proteins seem to be either heat-shock-specific or arsenite-specific. Studies using actinomycin D suggest that the response to arsenite is controlled mainly at the transcriptional level, for the 70-75-kDa group and 92-kDa proteins, but it seems that the other arsenite-induced proteins are subjected to transcriptional/translational control. In fact, results obtained by northern blotting, show that the mRNA coding for the 70-kDa stress protein is present only in stressed cells, whereas the 27-kDa-coding mRNA is present both in stressed and in unstressed cells. Inhibition of translation by cycloheximide has shown that heat-shock-induced-messengers are conserved under heat to be immediately translated upon removal of that inhibitor. Qualitatively similar results are obtained after prolonged treatments of T. pyriformis with arsenite and cycloheximide. The most striking difference between the responses of T. pyriformis to these two stress conditions is that arsenite does not repress normal protein synthesis so drastically as heat shock. In addition, our results suggest that some arsenite-induced messengers may be more stable than the corresponding heat-shock-induced messengers.

Regulation of gene expression in Tetrahymena pyriformis under heat-shock and during recovery

Proteins extracted from Tetrahymena pyriformis cells, which were shifted from 28 degrees C to 34 degrees C (heat-shocked) and then labelled with [35S]-methionine, were analysed by two-dimensional gel electrophoresis according to the method of O'Farrell. In the fluorogram obtained after 2.5 min heat-shock new polypeptide spots already appear in the region of 70-75 kDa and 25-29 kDa, indicating that the induction response is very rapidly triggered. Other newly induced proteins become more intense after 15 min and 30 min of heat-shock. After a 60-min treatment, the electrophoretic analysis shows the normal pattern of proteins resembling that of exponentially growing cells. Our experiments in vivo and in vitro also show that the messengers coding for stress proteins have a short lifetime, suggesting that modulation of the translation efficiency of heat-shock protein mRNAs is accompanied by an alteration in their biological stability. Studies using the inhibitor actinomycin D suggest that control of this response is exercise mainly at the transcriptional level for the 70-75-kDa and at the translational level for the 25-29-kDa heat-shock proteins. When T. pyriformis cells are allowed to recover at 28 degrees C the heat-shock proteins disappear completely and normal protein synthesis is completely re-established. Although the level of normal protein synthesis attained is lower, cells are able to recover completely from heat-shock when new mRNA synthesis is blocked by actinomycin D, indicating a conservation of normal pre-existing mRNAs during heat-shock.

Response of Tetrahymena pyriformis to stress induced by starvation

mRNA synthesis was studied in exponentially growing and starved Tetrahymena pyriformis. Poly(A)-containing RNAs separated from total RNA by affinity chromatography on oligo(dT)-cellulose were characterized by poly acrylamide gel electrophoresis; their template activity was assayed in a rabbit reticulocyte lysate system and their translation products were analysed using two-dimensional electrophoresis according to O'Farrell. Polysome profiles show that the bulk of ribosomes are in 80S monosomes in starved cells, whereas less than 8% are present in the form of monosomes in exponentially growing cells, the rest being engaged in polysomes. Polysomes are almost completely reformed 30 min after addition of enriched medium to suspensions of starved cells. This polysome reformation is dependent on mRNA synthesis since we have shown that it is inhibited by actinomycin D. Electrophoretic profiles of poly(A)-rich RNA isolated from cytoplasmic fractions of exponential and starved cells are indistinguishable except that in the latter state significant amounts of low-molecular-mass species are observed. Poly(A)-rich RNAs isolated from polysomal and non-polysomal (subpolysomal) fractions of exponential cells are equally able to promote protein synthesis. The corresponding poly(A)-rich RNAs isolated from starved cells also possess equal template activities which are, however, 15% lower than those of the poly(A)-rich RNAs of exponentially growing cells. We also present evidence that in the system used in vitro, polyadenylated RNA isolated from heavy polysomes of starved cells directs the synthesis of four sets of proteins with molecular masses around 100 kDa, 70 kDa, 50 kDa and 30 kDa. The former two groups of proteins are more abundant in the translation products of poly(A)-rich RNA of starved than of normal cells, whereas the latter two groups are present only in the translation products of poly(A)-rich RNA of starved Cells. The fluorograms of the translation products obtained in vitro from subpolysomal poly(A)-rich RNA are identical to those obtained from polysomal poly(A)-rich RNA. Studies on starved cells in vivo show that polypeptides of 100 kDa, 70 kDa and 38 kDa are more strongly labelled and also revealed the specific presence of 85 kDa, 55 kDa, 50 kDa and 25 kDa proteins. These results lead us to the conclusion that this microorganism responds to depleted environmental conditions by regulating gene expression at the transcriptional level, but also at the translational level.

Protein synthesis directed by polyadenylated and non-polyadenylated RNA isolated from membrane-bound and free polysomes of Tetrahymena pyriformis

Free and membrane-bound polysomes were prepared from the protozoa Tetrahymena pyriformis using a procedure which gives good recovery and practically no cross-contamination. Polysomes are intact as analysed by sedimentation analysis. Poly(A)-rich RNA and poly(A)-free RNA, isolated from both populations of polysomes, show similar electrophoretic patterns. These RNAs were translated in the rabbit reticulocyte lysate cell-free system and the translation products were analysed by one-dimensional and two-dimensional gel electrophoresis. The most striking differences were found in the two-dimensional electrophoretic analysis namely: (a) a group of polypeptides (10) is synthesized mainly on membrane-bound polysomes, (b) a second abundant group is synthesized mainly in free polysomes (c) and a third class of polypeptides is synthesized on both kinds of polysomes. Poly(A)-free RNAs, isolated from free polysomes, are also able to promote synthesis of some polypeptides. The results are discussed taking into account the fact that T. pyriformis is a non-secretory cell.