Localization of theBacillus subtilis murBGene within thedcwCluster Is Important for Growth and Sporulation

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Abstract
TheBacillus subtilis murBgene, encoding UDP-N-acetylenolpyruvoylglucosamine reductase, a key enzyme in the peptidoglycan (PG) biosynthetic pathway, is embedded in thedcw(for “division and cell wall”) cluster immediately upstream ofdivIB. Previous attempts to inactivatemurBwere unsuccessful, suggesting its essentiality. Here we show that the cell morphology, growth rate, and resistance to cell wall-active antibiotics ofmurBconditional mutants is a function of the expression level ofmurB. In one mutant, in whichmurBwas insertionally inactivated in a merodiploid bearing a second xylose-inducible PxylA-murBallele, DivIB levels were reduced and a normal growth rate was achieved only if MurB levels were threefold that of the wild-type strain. However, expression of an extra copy ofdivIBrestored normal growth at wild-type levels of MurB. In contrast, DivIB levels were normal in a second mutant containing an in-frame deletion ofmurB(ΔmurB) in the presence of the PxylA-murBgene. Furthermore, this strain grew normally with wild-type levels of MurB. During sporulation, the levels of MurB were highest at the time of synthesis of the spore cortex PG. Interestingly, the ΔmurBPxylA-murBmutant did not sporulate efficiently even at high concentrations of inducer. Since high levels of inducer did not interfere with sporulation of amurB+PxylA-murBstrain, it appears that ectopic expression ofmurBfails to support efficient sporulation. These data suggest that coordinate expression ofdivIBandmurBis important for growth and sporulation. The genetic context of themurBgene within thedcwcluster is unique to theBacillusgroup and, taken together with our data, suggests that in these species it contributes to the optimal expression of cell division and PG biosynthetic functions during both vegetative growth and spore development.