Analysis of the Che7 chemosensory system in Myxococcus xanthus.

摘要

Chemosensory systems in Myxococcus xanthus control a variety of cellular processes. The che7 cluster encodes six chemotaxis homologs as well as a homolog to a phycocyanobilin lyase ( cpc7) and a fatty acid desaturase (des7). Reverse transcriptase-based PCR analysis of the che7 gene cluster indicates that these eight genes are expressed as part of a single transcript. To determine the function of the Che7 system, mutants were constructed and analyzed for motility and developmental defects. While no motility defect was present, the mcp7, cpc7, and cheB7 mutants have severe defects in fruiting body formation and produce lower yields of viable spores. Due to the presence of the des homolog, we tested for defects in membrane composition. Fatty acid methyl esterification (FAME) analysis indicates that these mutants have increased levels of iso-branched fatty acids and decreased levels of unsaturated and saturated fatty acids. The cheB7 mutant also shows greater resistance to cold stress relative to the parent and decreased resistance to 37°C, whereas cheR7 displayed a reciprocal pattern of resistance to thermal stress. From these data, we conclude that the Che7 system regulates fatty acid composition of the membrane in response to stress. In addition to the altered fatty acid phenotype, the mcp7, cpc7 and cheB7 mutants were found to have strong defects in carotenoid regulation when compared to the parental strain. M. xanthus produces carotenoids in response to blue light through the regulation of the CarQRS pathway. Unlike mutations in the carQRS operon, these mutants are still able to repress carotenoid synthesis in the absence of light. Although the mechanism of control is not known, we conclude that the Che7 system regulates carotenoid levels post-translationally when exposed to blue light. Though a mechanism for coordinating the regulation of carotenoids and fatty acids is not known at this time, we hypothesize that Che7 directly regulates Des7 to produce fatty acids and Cpc7 which affects carotenoids, stabilizing the M. xanthus membrane when exposed to stress.