Poly-γ-glutamic acid production of Bacillus subtilis (natto) in the absence of DegQ: A gain-of-function mutation in yabJ gene

Poly-γ-glutamic acid (γPGA) production by Bacillus subtilis is regulated by the quorum sensing system where DegQ transmits the cell density signal to a DNA-binding protein DegU. A mutation suppressing the γPGA-negative phenotype of degQ gene knock-out mutant (ΔdegQ) was identified through whole genome sequencing. The mutation conferred an amino acid substitution of Ser103 to phenylalanine (S103F) in yabJ that belongs to the highly conserved YjgF/YER057c/UK114 family. Genetic experiments including LacZ-fusion assay of γPGA synthetic operon confirmed that the suppressor mutation (yabJ^S103F) was responsible for the recovery of γPGA production. The yabJ itself was not essential for the γPGA production and the mutant allele enabled γPGA production of the ΔdegQ strain even in the presence of wild type yabJ. Thus, yabJ^S103F was a dominant positive allele. degU-lacZ fusion gene was hyper-expressed in cells carrying the yabJ^S103F, but disruption of yabJ did not affect the transcription level of the degU-lacZ. These observations suggested that YabJ acquired a function to stimulate expression of degU by the S103F mutation which is involved in the regulation of γPGA synthesis.