Riboregulation of the bacterial actin-homolog MreB by DsrA small noncoding RNA.

The bacterial actin-homolog MreB is a key player in bacterial cell-wall biosynthesis and is required for the maintenance of the rod-like morphology of Escherichia coli. However, how MreB cellular levels are adjusted to growth conditions is poorly understood. Here, we show that DsrA, an E. coli small noncoding RNA (sRNA), is involved in the post-transcriptional regulation of mreB. DsrA is required for the downregulation of MreB cellular concentration during environmentally induced slow growth-rates, mainly growth at low temperature and during the stationary phase. DsrA interacts in an Hfq-dependent manner with the 5' region of mreB mRNA, which contains signals for translation initiation and thereby affects mreB translation and stability. Moreover, as DsrA is also involved in the regulation of two transcriptional regulators, σ(S) and the nucleoid associated protein H-NS, which negatively regulate mreB transcription, it also indirectly contributes to mreB transcriptional down-regulation. By using quantitative analyses, our results evidence the complexity of this regulation and the tangled interplay between transcriptional and post-transcriptional control. As transcription factors and sRNA-mediated post-transcriptional regulators use different timescales, we propose that the sRNA pathway helps to adapt to changes in temperature, but also indirectly mediates long-term regulation of MreB concentration. The tight regulation and fine-tuning of mreB gene expression in response to cellular stresses is discussed in regard to the effect of the MreB protein on cell elongation.

Multiple sequences and factors are involved in stability/degradation of Awnt-1, Awnt-5A and Awnt-5B mRNAs during axolotl development.

Following fertilization in amphibian, early cleavage stages are maternally controlled at a post-transcriptional level before initiation of zygotic transcriptions at the mid blastula transition (MBT). We document the expression levels of the axolotl Awnt-1, Awnt-5A and Awnt-5B genes as well as the adenylation states of their corresponding mRNAs from the end of oogenesis until the tailbud stages. Awnt-1/-5A RNAs are stable until MBT then degraded before gastrulation. Awnt-5B RNAs are degraded at fertilization and zygotically expressed after MBT with high level expression from gastrulation. Estimation of the poly(A) tail lengths reveals no direct link between deadenylation and degradation periods for each Awnt transcript. To investigate the molecular mechanisms involved in Awnt-1/-5A/-5B RNAs stability, synthetic full-length or 3' untranslated region (UTR) Awnt RNAs progressively deleted from their 3' end were microinjected in axolotl oocytes, unfertilized and fertilized eggs. We identified degrading and stabilizing sequences in the 3'UTR whose activities depend on the cellular context and are also modulated by the 5'UTR and coding sequence within each RNA. Using axolotl nuclear extracts from stage VI oocytes, we further produced evidence of destabilizing factors targeting the Awnt-5B RNAs. Altogether, these results show that oocyte maturation and late cleavages following MBT are two important periods when axolotl Wnt RNAs are highly regulated.

Characterization and expression of a maternal axolotl cyclin B1 during oogenesis and early development.

The M phase promoting factor (MPF) is a dimer composed of a catalytic Cdk1 subunit and a Cyclin B regulatory subunit. We have characterized a cDNA containing the entire coding sequence of an axolotl Cyclin B1 protein that is able to promote MPF activity when added to a fraction from prophase I oocytes that contains monomeric Cdk1. The axolotl cyclin B1 gene is expressed as a maternal mRNA in oocytes and early embryos. Its poly(A) tail length increases in metaphase II oocytes and then decreases regularly during the first embryonic cell cycles. Endogenous Cyclin B1 protein is first expressed during oocyte meiotic maturation. Its level oscillates after fertilization and is coordinated to the phosphorylation level of tyrosine 15 residue of Cdk1 (pTyr15), with both maxima preceding each cell division. As expected, when translated into microinjected oocytes, axolotl Cyclin B1 induces the resumption of meiosis. In electrically activated unfertilized eggs (UFE), Cyclin B1 and pTyr15 cyclic accumulations are observed with kinetics different from those of the early embryonic cycles. The axolotl embryo and UFE provide interesting in vivo comparative models for studying events controlling Cyclin B1 regulation during development.