Investigating the Roles of the Stk Locus in Development, Motility and Exopolysaccharide Production in Myxococcus Xanthus

Myxococcus xanthus, a Gram-negative bacterium with a developmental cycle, displays a type IV pili (TFP) mediated surface motility known as social (S) gliding. Beside the polarly localized TFP, the fibril or extracellular polysaccharide (EPS) is also required for S-motility to function. It is proposed that S-motility, along with the related bacterial twitching motility in other species, is powered by TFP retraction. EPS is proposed to anchor and trigger such retractions in M. xanthus. EPS production is known to be regulated by TFP and the Dif signal transduction pathway. Two genetic screens were performed previously to identify additional genes important for EPS production. The first was for the isolation of pilA suppressors, the second for the identification of mutants underproducing EPS in a difA suppressor background. Both screens identified transposon insertions at the stk locus. In particular, StkA, a DnaK homolog, was identified as a possible negative regulator of EPS production by a stkA transposon insertion that suppressed a pilA mutation. A stkB transposon insertion was found to have diminished EPS production in a difA suppressor background.

In this study, in-frame deletion mutants of the five genes at the stk locus, stkY, stkZ, stkA, stkB and stkC, were constructed and examined. In addition, mutations of rbp and bskL, two genes downstream of the stk locus, were constructed. Like transposon insertions, the stkA in-frame deletion resulted in overproduction of EPS. The stkB and to a less extent the stkC mutants underproduced EPS. Mutations in the other genes had no obvious effects on EPS production. Genetic epistasis suggests that StkA functions downstream of TFP and upstream of the Dif sensory proteins in EPS regulation in M. xanthus. Epistasis analysis involving stkB was inconclusive. It is unresolved whether StkB plays a role in the biosynthesis or the regulation of EPS production in M. xanthus.