Genetic Studies ofmrp, a Locus Essential for Cellular Aggregation and Sporulation ofMyxococcus xanthus

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Abstract
Under starvation conditions,Myxococcus xanthusundergoes a complex developmental process which includes cellular aggregation and sporulation. A transposon insertion mutant (the Tn5-Ω280 mutant) with defects in both aggregation and sporulation was analyzed in this study. The Tn5-Ω280 mutant was found to have a disrupted NtrC-like response regulator designatedMyxococcusregulatory protein B (mrpB). Further sequencing analyses revealed a histidine kinase homolog (mrpA) immediately upstream ofmrpBand a cyclic AMP receptor protein-like transcriptional regulator (mrpC) downstream ofmrpB. In-frame deletion analyses revealed that both themrpBandmrpCgenes were required for cellular aggregation and sporulation but that onlymrpAwas required for sporulation only. Site-specific mutagenesis of the putative phosphorylation site of MrpB, D58, showed that a D58A mutation caused defects in both aggregation and sporulation but that a D58E mutation resulted in only a sporulation defect. Further genetic and molecular analyses with reporter genes and reverse transcription-PCR indicated thatmrpAandmrpBare cotranscribed but thatmrpCis transcribed independently and that all of these genes are developmentally regulated. In addition, MrpB is essential for transcription ofmrpCand MrpC regulates its own transcription. These data indicate that Mrp proteins are important components required forM. xanthusdevelopment. The complicated interaction between Mrp proteins may play an important role in regulating developmental gene expression inM. xanthus.