The sdeK gene is essential to the Myxococcus xanthus developmental process. We reported previously, based on sequence analysis (A. G. Garza, J. S. Pollack, B. Z. Harris, A. Lee, I. M. Keseler, E. F. Licking, and M. Singer, J. Bacteriol. 180:4628-4637, 1998), that SdeK appears to be a histidine kinase. In the present study, we have conducted both biochemical and genetic analyses to test the hypothesis that SdeK is a histidine kinase. An SdeK fusion protein containing an N-terminal polyhistidine tag (His-SdeK) displays the biochemical characteristics of a histidine kinase. Furthermore, histidine 286 of SdeK, the putative site of phosphorylation, is required for both in vitro and in vivo protein activity. The results of these assays have led us to conclude that SdeK is indeed a histidine kinase. The developmental phenotype of a ⌬sdeK1 strain could not be rescued by codevelopment with wild-type cells, indicating that the defect is not due to the mutant's inability to produce an extracellular signal. Furthermore, the ⌬sdeK1 mutant was found to produce both A-and C-signal, based on A-factor and codevelopment assays with a csgA mutant, respectively. The expression patterns of several Tn5lacZ transcriptional fusions were examined in the ⌬sdeK1-null background, and we found that all C-signaldependent fusions assayed also required SdeK for full expression. Our results indicate that SdeK is a histidine kinase that is part of a signal transduction pathway which, in concert with the C-signal transduction pathway, controls the activation of developmental-gene expression required to progress past the aggregation stage.

In response to starvation, Myxococcus xanthus undergoes a multicellular developmental process that produces a dome‐shaped fruiting body structure filled with differentiated cells called myxospores. Two insertion mutants that block the final stages of fruiting body morphogenesis and reduce sporulation efficiency were isolated and characterized. DNA sequence analysis revealed that the chromosomal insertions are located in open reading frames ORF2 and asgE, which are separated by 68 bp. The sporulation defect of cells carrying the asgE insertion can be rescued phenotypically when co‐developed with wild‐type cells, whereas the sporulation efficiency of cells carrying the ORF2 insertion was not improved when mixed with wild‐type cells. Thus, the asgE insertion mutant appears to belong to a class of developmental mutants that are unable to produce cell–cell signals required for M. xanthus development, but they retain the ability to respond to them when they are provided by wild‐type cells. Several lines of evidence indicate that asgE cells fail to produce normal levels of A‐factor, a cell density signal. A‐factor consists of a mixture of heat‐stable amino acids and peptides, and at least two heat‐labile extracellular proteases. The asgE mutant yielded about 10‐fold less heat‐labile A‐factor and about twofold less heat‐stable A‐factor than wild‐type cells, suggesting that the primary defect of asgE cells is in the production or release of heat‐labile A‐factor.

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