A-signalling and the cell density requirement for Myxococcus xanthus development

Mutations in any of three asg (A-signalling) loci cause fruiting body development of Myxococcus xanthus to arrest at about the 2-h stage. Development can be restored to asg mutants by the addition of conditioned buffer in which wild-type cells have been developing or of A-factor purified from the conditioned buffer. Two forms of A-factor have been identified: heat-stable A-factor, which is composed of amino acids and peptides, and heat-labile A-factor, which consists of at least two proteases. A-factor is found in conditioned buffer in rough proportion to the cell density. As decreasing amounts of either form of A-factor are added, the developmental response of asg cells decreases until a threshold concentration is reached, below which no response is detected. In addition, wild-type cells fail to develop when their density is decreased below the point at which the level of A-factor is predicted to fall short of this threshold. The development of low-density asg+ cells can, however, be restored by the addition of either form of A-factor. These experiments show that A-factor is important for the development of wild-type cells. Moreover, the development of an asgB mutant that produces 5 to 10% the wild-type level of A-factor can be restored when the cell density is increased 10-fold above the standard density. We propose that the A-signal is used by M. xanthus to specify the minimum cell density required for the initiation of development. Differences in the response to A-factor between different asg mutants suggest that the different asg loci govern A-factor production in diverse ways.