Transcriptomic analysis of Myxococcus xanthus csgA, fruA, and mrpC mutants reveals extensive and diverse roles of key...

The bacterium Myxococcus xanthus provides an important multicellular model for understanding stress responses. The regulatory proteins CsgA, FruA, and MrpC are essential to survive prolonged starvation by forming fruiting bodies, which are mounds containing hardy round spores formed from vegetative rods, but the genome-wide pathways affected by these proteins remain poorly understood. Only a fruA mutant transcriptome and MrpC ChIP-seq have been reported. We describe RNA-seq transcriptome analysis of csgA, fruA, and mrpC mutants relative to a wild-type laboratory strain midway during the starvation-induced developmental process, when mounds, but not spores, have formed.

We show that CsgA, FruA, and MrpC broadly impact developmental gene expression, with over 60% of the genes differentially expressed in one or more mutants. Building upon previous investigations, we found that strongly regulated genes in the mrpC mutant correlate with MrpC DNA-binding sites located ~ 80 bp upstream of transcriptional start sites. We also confirmed that FruA directly or indirectly regulates many genes negatively, as well as many others positively. CsgA regulates indirectly and its strongest effects are positive. MrpC strongly stimulates fruA transcription and FruA accumulation, impacting many genes, but our results reveal that MrpC is also a strong negative or positive regulator of hundreds of genes independently of FruA. Indeed, we observed nearly every possible pattern of coregulation, unique regulation, and counterregulation by comparing the wild-type and mutant transcriptomes, indicating diverse roles of CsgA, FruA, and MrpC in the developmental gene regulatory network. The genes most strongly regulated were coregulated in two or three of the mutants. Each set of genes exhibiting differential expression in one or more mutants was analyzed for enrichment of gene ontology (GO) terms or KEGG pathways, and predicted protein-protein interactions. These analyses highlighted enrichment of pathways involved in cellular signaling, protein synthesis, energetics, and envelope function. In particular, we describe how CsgA, FruA, and MrpC control production of ribosomes, lipid signals, and peptidoglycan intermediates during development.

By comparing wild-type and mutant transcriptomes midway in development, this study documents individual and coordinate regulation of crucial pathways by CsgA, FruA, and MrpC, providing a valuable resource for future investigations.