Proteins that rescue A-signal-defective mutants of Myxococcus xanthus

The asg mutants of Myxococcus xanthus are defective in the production of an extracellular substance, called A-factor, that is required for expression of a set of fruiting body-specific genes. A-factor is released by wild-type cells (asg+) after 1 to 2 h of development. When A-factor is added to asg mutant cells, it restores expression of their A-factor-dependent genes. Rescue of beta-galactosidase production in an asg mutant carrying the A-factor-dependent lacZ transcriptional fusion (omega 4521) was used to assay A-factor activity. According to this assay, two types of substances with A-factor activity are present in conditioned medium. One type is heat stable and of low molecular weight; the other is heat labile and of high molecular weight. An approximately 27-kDa protein with heat-labile A-factor activity was purified from conditioned medium. The purified protein has proteolytic activity as well as A-factor activity. The substrate specificity of the 27-kDa protease resembles that of trypsin. A smaller protein with both heat-labile A-factor activity and proteolytic activity was identified. Its substrate specificity differs from that of the 27-kDa protein. In addition, trypsin and other proteases were found to have heat-labile A-factor activity. Trypsin inhibitory protein from soybeans neutralizes the A-factor activity of trypsin in parallel with its neutralization of protease activity, showing that the proteolytic activity of trypsin is necessary for its A-factor activity. The 27-kDa protein rescues the aggregation and sporulation defects of an asgB mutant in submerged culture as well as its ability to express beta-galactosidase from an asg-dependent lac fusion.