Characterization of new genes required for adventurous gliding motility of Myxococcus xanthus

Myxococcus xanthus is a Gram-negative soil-dwelling bacterium that glides across solid surfaces using two genetically distinct mechanisms. Rafts of cells at a high density move by Social (S) gliding, which requires type IV pili, lipopolysaccharide (LPS) and polysaccharide fibrils. Individual cells at low density move by Adventurous (A) gliding, which requires membrane-associated protein complexes. When starved M. xanthus cells enter a developmental cycle initiated by the aggregation of thousands of cells and culminates in the production of a fruiting body containing heat-resistant spores. The aglU (a&barbelow;dventurous gliding U&barbelow;nknown) gene of M. xanthus encodes a protein that is essential for A-motility and a late stage of development when non-refractile spores become refractile, an event coincident with heat-sensitive spores becoming heat-resistant. AglU is a lipoprotein similar to the WD-repeat TolB protein of Escherichia coli, which is part of the envelope-spanning multi-protein TolQ-TolR-TolA-TolB complex involved in energy-dependent biopolymer transport. Strains with mutations in five additional genes (tolQ1, tolQ2, tolR1 tolR2, tolB2) have the same A-motility and developmental phenotypes as the aglU mutant. These genes encode proteins that are homologs of the other Tol proteins. Gliding motility is abolished in the double mutant strains aglU to1B2 and aglU tolQ2, however the aglU tolQ1 double mutant strain has the identical A-motility phenotype as the aglU parent strain, suggesting that two Tol-like complexes—an AglU-Tol complex and a TolB-Tol complex, are involved in adventurous gliding in M. xanthus. A mutation in the glucose inhibited division (gidA) gene, located upstream from, and transcribed with aglU during development, does not affect A-motility, yet abolishes development. A low molecular weight extracellular inhibitor isolated from gidA mutant cells is able to block development of other M. xanthus strains. This inhibitor is predicted to be transported by the AglU-Tol-like complex because a strain carrying mutations in both gidA and aglU does not export the inhibitor.