FruA, a putative transcription factor essential for the development of Myxococcus xanthus

A new developmental gene, fruA, of Myxococcus xanthus was cloned using a one-step cloning vector, TnV. DNA sequencing of the wild-type allele of the fruA gene indicated that the fruA gene encodes a protein of 229 amino acid residues with a calculated molecular weight of 24672. The deduced amino acid sequence of FruA protein showed similarity to those of many bacterial regulatory proteins carrying a DNA-binding helix-turn-helix motif. The transcription-initiation site of the fruA gene was determined by a primer-extension experiment. Development of M. xanthus cells with a disrupted fruA gene stopped at the stage of mound formation. Although cells were able to aggregate to form mounds, myxospores were not formed. By Northern and Western blot analysis, it was found that the fruA expression was not detected during vegetative growth but initiated at around 6 h and reached the highest level at 12 h after the onset of development. Expression of the fruA gene was dependent on the expression of asg, bsg, csg, dsg, and esg genes, indicating that a series of intercellular signalling is necessary for the expression of the fruA gene. The effects of the fruA mutation on beta-galactosidase expression of various developmentally regulated genes fused with the lacZ gene were analysed; three developmental lacZ fusions (omega 4469, omega 4273 and omega 4500) were either poorly induced or not induced at all, while three other lacZ fusions (omega 4408, omega 4521 and omega 4455) expressed at the early stage of development were normally induced but were unable to be repressed at a later stage of development as in the wild-type strain. Interestingly, in the fruA mutant, tps (the gene for protein S) was not activated. From these results together with analysis of the amino acid sequence of FruA, we propose that FruA is a putative transcription factor required for the development of M.xanthus.