csgA expression entrains Myxococcus xanthus development

Abstract

The development cycle of the myxobacterium Myxococcus xanthus consists of three partially overlapping morphological stages referred to as rippling, fruiting body formation, and sporulation, all of which are absent in csgA null mutants. The CsgA gene product is an extracellular protein, referred to as the C signal, which is essential for developmental cell-cell interactions. csgA expression increases throughout development, reaching its peak during sporulation. CsgA was made limiting for development by constructing nested deletions upstream from the csgA gene, which resulted in reduced csgA expression. Successively larger deletions resulted in termination of development at earlier and earlier stages, with rippling requiring approximately 20% maximum csgA expression, fruiting body formation requiring approximately 30% expression, and sporulation requiring 82% expression. Conversely, artificial induction of csgA also induced development provided nutrients were limiting. These results suggest that steady increases in CsgA over the course of development entrain the natural sequence of morphological events. The csgA upstream region appears to process information concerning the levels of nutrients, peptidoglycan components, and the B signal. In the absence of nutrients, a region extending 400 bp upstream from the start site of transcription was necessary for development and maximal csgA expression. In the presence of low levels of nutrients, a region extending approximately 930 bp upstream was essential for the same tasks. It appears that the upstream region extending from -400 to -930 stimulates csgA expression in the presence of excess carbon, nitrogen, and phosphate, thereby allowing development to go to completion.