ABSTRACTKnowledge of how microorganisms respond and adapt to low-pressure (LP) environments is limited. Previously,Bacillus subtilisstrain WN624 was grown at the near-inhibitory LP of 5 kPa for 1,000 generations and strain WN1106, which exhibited increased relative fitness at 5 kPa, was isolated. Genomic sequence differences between ancestral strain WN624 and LP-evolved strain WN1106 were identified using whole-genome sequencing. LP-evolved strain WN1106 carried amino acid-altering mutations in the coding sequences of only seven genes (fliI,parC,ytoI,bacD,resD,walK, andyvlD) and a single 9-nucleotide in-frame deletion in thernjBgene that encodes RNase J2, a component of the RNA degradosome. By using a collection of frozen stocks of the LP-evolved culture taken at 50-generation intervals, it was determined that (i) the fitness increase at LP occurred rapidly, while (ii) mutation acquisition exhibited complex kinetics. A knockout mutant ofrnjBwas shown to increase the competitive fitness ofB. subtilisat both LP and standard atmospheric pressure.