The Hsp70-like StkA functions between T4P and Dif signaling proteins as a negative regulator of exopolysaccharide in Myxococcus xanthus

Myxococcus xanthus displays a form of surface motility known as social (S) gliding. It is mediated by the type IV pilus (T4P) and requires the exopolysaccharide (EPS) to function. It is clear that T4P retraction powers S motility. EPS on a neighboring cell or deposited on a gliding surface is proposed to anchor the distal end of a pilus and trigger T4P retraction at its proximal end. Inversely, T4P has been shown to regulate EPS production upstream of the Dif signaling pathway. Here we describe the isolation of two Tn insertions at the stk locus which had been known to play roles in cellular cohesion and formation of cell groups. An insertion in stkA (MXAN_3474) was identified based on its ability to restore EPS to a pilA deletion mutant. The stkA encodes a DnaK or Hsp70 homolog and it is upstream of stkB (MXAN_3475) and stkC (MXAN_3476). A stkB insertion was identified in a separate genetic screen because it eliminated EPS production of an EPS+ parental strain. Our results with in-frame deletions of these three stk genes indicated that the stkA mutant produced increased level of EPS while stkB and stkC mutants produced less EPS relative to the wild type. S motility and developmental aggregation were affected by deletions of stkA and stkB but only minimally by the deletion of stkC. Genetic epistasis indicated that StkA functions downstream of T4P but upstream of the Dif proteins as a negative regulator of EPS production in M. xanthus.