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Nutrient-regulated proteolysis of MrpC halts expression of genes important for commitment to sporulation during Myxococcus xanthus development.

ABSTRACT: Starved Myxococcus xanthus cells glide to aggregation centers and form fruiting bodies in which rod-shaped cells differentiate into ovoid spores. Commitment to development was investigated by adding nutrients at specific times after starvation and determining whether development halted or proceeded. At 24 h poststarvation, some rod-shaped cells were committed to subsequent shape change and to becoming sonication-resistant spores, but nutrients caused partial disaggregation of fruiting bodies. By 30 h poststarvation, 10-fold more cells were committed to becoming sonication-resistant spores, and compact fruiting bodies persisted after nutrient addition. During the critical period of commitment around 24 to 30 h poststarvation, the transcription factors MrpC and FruA cooperatively regulate genes important for sporulation. FruA responds to short-range C-signaling, which increases as cells form fruiting bodies. MrpC was found to be highly sensitive to nutrient-regulated proteolysis both before and during the critical period of commitment to sporulation. The rapid turnover of MrpC upon nutrient addition to developing cells halted expression of the dev operon, which is important for sporulation. Regulated proteolysis of MrpC appeared to involve ATP-independent metalloprotease activity and may provide a mechanism for monitoring whether starvation persists and halting commitment to sporulation if nutrients reappear.

SUBMITTER: Rajagopalan R

PROVIDER: S-EPMC4135671 | biostudies-literature | 2014 Aug

REPOSITORIES: biostudies-literature

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Nutrient-regulated proteolysis of MrpC halts expression of genes important for commitment to sporulation during Myxococcus xanthus development.

Rajagopalan Ramya R Kroos Lee L

Journal of bacteriology 20140516 15

Starved Myxococcus xanthus cells glide to aggregation centers and form fruiting bodies in which rod-shaped cells differentiate into ovoid spores. Commitment to development was investigated by adding nutrients at specific times after starvation and determining whether development halted or proceeded. At 24 h poststarvation, some rod-shaped cells were committed to subsequent shape change and to becoming sonication-resistant spores, but nutrients caused partial disaggregation of fruiting bodies. By ...[more]

PMID: 24837289

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Project description:BackgroundMyxococcus xanthus is a bacterium that undergoes multicellular development when starved. Cells move to aggregation centers and form fruiting bodies in which cells differentiate into dormant spores. MrpC appears to directly activate transcription of fruA, which also codes for a transcription factor. Both MrpC and FruA are crucial for aggregation and sporulation. The two proteins bind cooperatively in promoter regions of some developmental genes.ResultsChromatin immunoprecipitation followed by DNA sequencing (ChIP-seq) and bioinformatic analysis of cells that had formed nascent fruiting bodies revealed 1608 putative MrpC binding sites. These sites included several known to bind MrpC and they were preferentially distributed in likely promoter regions, especially those of genes up-regulated during development. The up-regulated genes include 22 coding for protein kinases. Some of these are known to be directly involved in fruiting body formation and several negatively regulate MrpC accumulation. Our results also implicate MrpC as a direct activator or repressor of genes coding for several transcription factors known to be important for development, for a major spore protein and several proteins important for spore formation, for proteins involved in extracellular A- and C-signaling, and intracellular ppGpp-signaling during development, and for proteins that control the fate of other proteins or play a role in motility. We found that the putative MrpC binding sites revealed by ChIP-seq are enriched for DNA sequences that strongly resemble a consensus sequence for MrpC binding proposed previously. MrpC2, an N-terminally truncated form of MrpC, bound to DNA sequences matching the consensus in all 11 cases tested. Using longer DNA segments containing 15 of the putative MrpC binding sites from our ChIP-seq analysis as probes in electrophoretic mobility shift assays, evidence for one or more MrpC2 binding site was observed in all cases and evidence for cooperative binding of MrpC2 and FruA was seen in 13 cases.ConclusionsWe conclude that MrpC and MrpC2 bind to promoter regions of hundreds of developmentally-regulated genes in M. xanthus, in many cases cooperatively with FruA. This binding very likely up-regulates protein kinases, and up- or down-regulates other proteins that profoundly influence the developmental process.

Project description:Upon depletion of nutrients, Myxococcus xanthus forms mounds on a solid surface. The differentiation of rod-shaped cells into stress-resistant spores within mounds creates mature fruiting bodies. The developmental process can be perturbed by the addition of nutrient medium before the critical period of commitment to spore formation. The response was investigated by adding a 2-fold dilution series of nutrient medium to starving cells. An ultrasensitive response was observed, as indicated by a steep increase in the spore number after the addition of 12.5% versus 25% nutrient medium. The level of MrpC, which is a key transcription factor in the gene regulatory network, correlated with the spore number after nutrient medium addition. The MrpC level decreased markedly by 3 h after adding nutrient medium but recovered more after the addition of 12.5% than after 25% nutrient medium addition. The difference in MrpC levels was greatest midway during the period of commitment to sporulation, and mound formation was restored after 12.5% nutrient medium addition but not after adding 25% nutrient medium. Although the number of spores formed after 12.5% nutrient medium addition was almost normal, the transcript levels of "late" genes in the regulatory network failed to rise normally during the commitment period. However, at later times, expression from a reporter gene fused to a late promoter was higher after adding 12.5% than after adding 25% nutrient medium, consistent with the spore numbers. The results suggest that a threshold level of MrpC must be achieved in order for mounds to persist and for cells within to differentiate into spores.IMPORTANCE Many signaling and gene regulatory networks convert graded stimuli into all-or-none switch-like responses. Such ultrasensitivity can produce bistability in cell populations, leading to different cell fates and enhancing survival. We discovered an ultrasensitive response of M. xanthus to nutrient medium addition during development. A small change in nutrient medium concentration caused a profound change in the developmental process. The level of the transcription factor MrpC correlated with multicellular mound formation and differentiation into spores. A threshold level of MrpC is proposed to be necessary to initiate mound formation and create a positive feedback loop that may explain the ultrasensitive response. Understanding how this biological switch operates will provide a paradigm for the broadly important topic of cellular behavior in microbial communities.

Project description:UnlabelledIn the bacterium Myxococcus xanthus, starvation triggers the formation of multicellular fruiting bodies containing thousands of stress-resistant spores. Recent work showed that fruiting body development is regulated by a cascade of transcriptional activators called enhancer binding proteins (EBPs). The EBP Nla6 is a key component of this cascade; it regulates the promoters of other EBP genes, including a downstream-functioning EBP gene that is crucial for sporulation. In recent expression studies, hundreds of Nla6-dependent genes were identified, suggesting that the EBP gene targets of Nla6 may be part of a much larger regulon. The goal of this study was to identify and characterize genes that belong to the Nla6 regulon. Accordingly, a direct repeat [consensus, C(C/A)ACGNNGNC] binding site for Nla6 was identified using in vitro and in vivo mutational analyses, and the sequence was subsequently used to find 40 potential developmental promoter (88 gene) targets. We showed that Nla6 binds to the promoter region of four new targets (asgE, exo, MXAN2688, and MXAN3259) in vitro and that Nla6 is important for their normal expression in vivo. Phenotypic studies indicate that all of the experimentally confirmed targets of Nla6 are primarily involved in sporulation. These targets include genes involved in transcriptional regulation, cell-cell signal production, and spore differentiation and maturation. Although sporulation occurs late in development, all of the developmental loci analyzed here show an Nla6-dependent burst in expression soon after starvation is induced. This finding suggests that Nla6 starts preparing cells for sporulation very early in the developmental process.ImportanceBacterial development yields a remarkable array of complex multicellular forms. One such form, which is commonly found in nature, is a surface-associated aggregate of cells known as a biofilm. Mature biofilms are structurally complex and contain cells that are highly resistant to antibacterial agents. When starving, the model bacterium Myxococcus xanthus forms a biofilm containing a thin mat of cells and multicellular structures that house a highly resistant cell type called a myxospore. Here, we identify the promoter binding site of the transcriptional activator Nla6, identify genes in the Nla6 regulon, and show that several of the genes in the Nla6 regulon are important for production of stress-resistant spores in starvation-induced M. xanthus biofilms.

Project description:Myxococcus xanthus undergoes multicellular development when starved. Thousands of rod-shaped cells coordinate their movements and aggregate into mounds in which cells differentiate into spores. Mutations in the dev operon impair development. The dev operon encompasses a clustered regularly interspaced short palindromic repeat-associated (CRISPR-Cas) system. Null mutations in devI, a small gene at the beginning of the dev operon, suppress the developmental defects caused by null mutations in the downstream devR and devS genes but failed to suppress defects caused by a small in-frame deletion in devT We provide evidence that the original mutant has a second-site mutation. We show that devT null mutants exhibit developmental defects indistinguishable from devR and devS null mutants, and a null mutation in devI suppresses the defects of a devT null mutation. The similarity of DevTRS proteins to components of the CRISPR-associated complex for antiviral defense (Cascade), together with our molecular characterization of dev mutants, support a model in which DevTRS form a Cascade-like subcomplex that negatively autoregulates dev transcript accumulation and prevents DevI overproduction that would strongly inhibit sporulation. Our results also suggest that DevI transiently inhibits sporulation when regulated normally. The mechanism of transient inhibition may involve MrpC, a key transcription factor, whose translation appears to be weakly inhibited by DevI. Finally, our characterization of a devI devS mutant indicates that very little exo transcript is required for sporulation, which is surprising since Exo proteins help form the polysaccharide spore coat.IMPORTANCE CRISPR-Cas systems typically function as adaptive immune systems in bacteria. The dev CRISPR-Cas system of M. xanthus has been proposed to prevent bacteriophage infection during development, but how dev controls sporulation has been elusive. Recent evidence supported a model in which DevR and DevS prevent overproduction of DevI, a predicted 40-residue inhibitor of sporulation. We provide genetic evidence that DevT functions together with DevR and DevS to prevent DevI overproduction. We also show that spores form about 6 h earlier in mutants lacking devI than in the wild type. Only a minority of natural isolates appear to have a functional dev promoter and devI, suggesting that a functional dev CRISPR-Cas system evolved recently in niches where delayed sporulation and/or protection from bacteriophage infection proved advantageous.

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Nutrient-regulated proteolysis of MrpC halts expression of genes important for commitment to sporulation during Myxococcus xanthus development.

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Nutrient-regulated proteolysis of MrpC halts expression of genes important for commitment to sporulation during Myxococcus xanthus development.

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