| Myxococcus xanthus is a Gram-negative social prokaryote that utilizes gliding motility. Adventurous (A) and social (S) gliding motility are dependent on the expression of over 200 genes that encode motor proteins, sensory systems and extracellular matrix regulators. A and S-motility are required for a complex developmental program that allows M. xanthus to survive conditions of hardship such as heat, desiccation and nutrient starvation. Coordination of these two motility systems is contingent on the function of a Ras-like GTPase, MglA. MglA activity requires GTP-nucleotide binding and hydrolysis, which depends upon conserved residues in consensus motifs PM1, G2 and PM3. GTP hydrolysis is required in order to control downstream signaling proteins in both A and S motors. Although the PM3 consensus has some conservation to Ras, MglA has a threonine present at position 78, and engineering PM3 to match the Ras T78D consensus results in stable, but nonfunctional MglA.;An mglA allele, mglA8, is nonmotile (A-S-). The S-motility phenotype can be partially suppressed by a second-site mutation in the masK gene. MasK is a known serine-threonine-tyrosine protein kinase required for S-motility. Upstream of masK are two genes, masA and masB, which are cotranscribed in one polycistronic mRNA with masK. MasA appears to be an integral inner membrane protein while MasB is a putative lipoprotein. Deletion of the masABK operon specifically affects S-motility, swarming, gliding velocity and development. The deletion in masABK abolished pilA expression without causing detectable mutation at the pilA locus. Addition of a wild-type (WT) copy of pilA restored social motility to nearly 40% of WT, while swarming was restored to 78% of WT. Development was restored only in a ΔmasABK strain complemented with pilA and masABK. Epigenetic silencing or second-site mutations may be responsible for an inability of masABK to complement the masABK deletion. |
