| Myxococcus xanthus, a gram-negative soil bacterium, exhibits striking social behavior. When food is plentiful Myxococcus xanthus forms swarms of vegetative cells; when food is scarce, the cells aggregate to form fruiting bodies and ultimately, resistant myxospores.; At least five cell-cell signals (A, B, C, D & E) are used to coordinate this complex developmental cycle. A-signal, the cellular density signal, consists of a mixture of amino acids. At least four genes encoding regulatory proteins are involved in its generation. One of these genes, asgA, is the subject of this study. asgA encodes a histidine kinase homologous to those found in two-component regulatory systems. Its receiver domain is followed by a kinase domain, and it lacks input and output domains necessary for signal sensing and gene regulation. AsgA probably functions in a phosphorelay pathway in which other proteins provide input and output functions.; A suppressor analysis of a mutant asgA was undertaken to find other proteins that are important in the A-signal production pathway. Twenty-two independent suppressors obtained by UV mutagenesis have direct spore counts three to four times greater than the asgA mutant, and three suppressors have viable spore counts one order of magnitude greater than the asgA mutant. None of these suppressors produce wild type levels of A-signal. Five independent Tn5 insertion suppressors restore the fruiting body ability to the asgA mutant. Two of the suppressor mutations were found to be in the suppressor of protease ( spd) locus, in the spdR and spdS genes, which encode a response regulator and a histidine kinase, respectively. A third insertion was found in the early sporulation locus (esp) within the espA gene which encodes a histidine kinase that regulates the timing of sporulation initiation. The last two insertions, named suppressor of asgA (saa), were found in two novel ORFs. saaG encodes a putative sensor histidine kinase. saaP encodes a putative protease. All five Tn5 insertions suppress the asgB480 mutation to varied degrees, suggesting that they may function downstream of asgB. These suppressors, with the exception of saaP, do not change the developmental phenotype of the wild type strain DK1622. |
