Plant-specific environmental and developmental signals regulate the mismatch repair protein MSH6 in Arabidopsis thaliana.

The DNA mismatch repair (MMR) is a postreplicative system that guarantees genomic stability by correcting mispaired and unpaired nucleotides. In eukaryotic nuclei, MMR is initiated by the binding of heterodimeric MutS homologue (MSH) complexes to the DNA error or lesion. Among these proteins, MSH2-MSH6 is the most abundant heterodimer. Even though the MMR mechanism and proteins are highly conserved throughout evolution, physiological differences between species can lead to different regulatory features. Here, we investigated how light, sugar, and/or hormones modulate Arabidopsis thaliana MSH6 expression pattern. We first characterized the promoter region of MSH6. Phylogenetic shadowing revealed three highly conserved regions. These regions were analyzed by the generation of deletion constructs of the MSH6 full-length promoter fused to the ß-glucuronidase (GUS) gene. Combined, our in silico and genetic analyses revealed that a 121-bp promoter fragment was necessary for MSH6 expression and contained potential cis-acting elements involved in light- and hormone-responsive gene expression. Accordingly, light exposure or sugar treatment of four-day old A. thaliana seedlings triggered an upregulation of MSH6 in shoot and root apical meristems. Appropriately, MSH6 was also induced by the stem cell inducer WUSCHEL. Further, the stimulatory effect of light was dependent on the presence of phyA. In addition, treatment of seedlings with auxin or cytokinin also caused an upregulation of MSH6 under darkness. Consistent with auxin signals, MSH6 expression was suppressed in the GATA23 RNAi line compared with the wild type. Our results provide evidence that endogenous factors and environmental signals controlling plant growth and development regulate the MSH6 protein in A. thaliana.

The multifarious MerR family of transcriptional regulators.

The MerR family of transcriptional regulators includes a variety of bacterial cytoplasmic proteins that respond to a wide range of signals, including toxins, metal ions, and endogenous metabolites. Its best-characterized members share similar structural and functional features with the family founder, the mercury sensor MerR, although most of them do not respond to metal ions. The group of "canonical" MerR homologs displays common molecular mechanisms for controlling the transcriptional activation of their target genes in response to inducer signals. This includes the recognition of distinctive operator sequences located at suboptimal σ70 -dependent promoters. Interestingly, an increasing number of proteins assigned to the MerR family based on their DNA-binding domain do not match in structure, sequence, or mode of action with any of the canonical MerR-like regulators. Here, we analyzed several members of the family, including this last group. Based on a phylogenetic analysis, and similarities in structural/functional features and position of their target operators relative to the promoter elements, we propose to assign these "atypical/divergent" MerR regulators to a phylogenetically separated group. These atypical/divergent homologs represent a new class of transcriptional regulators with novel regulatory mechanisms.

Balance between bacterial extracellular matrix production and intramacrophage proliferation by a Salmonella-specific SPI-2-encoded transcription factor.

Biosynthesis and secretion of a complex extracellular matrix (EM) is a hallmark of Salmonella biofilm formation, impacting on its relationship with both the environment and the host. Cellulose is a major component of Salmonella EM. It is considered an anti-virulence factor because it interferes with Salmonella proliferation inside macrophages and virulence in mice. Its synthesis is stimulated by CsgD, the master regulator of biofilm formation in enterobacteria, which in turn is under the control of MlrA, a MerR-like transcription factor. In this work, we identified a SPI-2-encoded Salmonella-specific transcription factor homolog to MlrA, MlrB, that represses transcription of its downstream gene, orf319, and of csgD inside host cells. MlrB is induced in laboratory media mimicking intracellular conditions and inside macrophages, and it is required for intramacrophage proliferation. An increased csgD expression is observed in the absence of MlrB inside host cells. Interestingly, inactivation of the CsgD-controlled cellulose synthase-coding gene restored intramacrophage proliferation to rates comparable to wild-type bacteria in the absence of MlrB. These data indicate that MlrB represses CsgD expression inside host cells and suggest that this repression lowers the activation of the cellulose synthase. Our findings provide a novel link between biofilm formation and Salmonella virulence.