The respiratory lipoquinone, menaquinone, functions as an inducer of genes regulated by the Mycobacterium smegmatis repressor MSMEG_2295.

A previous study reported that the Mycobacterium smegmatis (Msm) protein MSMEG_2295 is a repressor controlling the expression of several genes, including that for MSMEG_5125, a putative isoprenoid binding protein belonging to the YceI family, and DinB2, a DNA damage repair enzyme. This repressor is encoded by the first gene of the operon that also expresses the gene for DinB2. Targeted inhibition of MSMEG_5125 using CRISPRi technology resulted in a significant loss of Msm's respiratory activity and viability. Since this protein has been predicted to be an isoprenoid binding protein, we suspected a role of menaquinones, which are isoprenoid naphthoquinones, in the observed phenomenon. Accordingly, we tested whether MSMEG_5125's deficiency-induced lethality could be reversed by adding menaquinone. The result was positive, implying cooperation between MSMEG_5125 and menaquinone in bringing about respiration. Inhibition of MSMEG_5125 expression led to the induction of MSMEG_0089 and 2296, two hallmark genes of the MSMEG_2295 regulon. This result suggests that when MSMEG_5125 becomes limiting, a feedback-loop derepresses the MSMEG_2295 regulon genes, including its own. Interestingly, menaquinone functioned as an inducer of MSMEG_5125, indicating that it is likely to mediate the feedback mechanism. This result also strengthens our hypothesis that the functions of menaquinone and MSMEG_5125 are interrelated. Menaquinone also induced the MSMEG_2295-controlled operon MSMEG_2295-2294 (dinB2) not induced following the inactivation of MSMEG_5125. Therefore, the activation mechanism of MSMEG_2295-regulated genes may not be the same for all, although derepression is likely to be a common feature. In vitro, menaquinone abolished MSMEG_2295's DNA binding activity by interacting with it, confirming its role as an inducer. Therefore, a menaquinone-MSMEG_5125-regulated gene expression circuit controls Msm respiration and possibly oxidative stress-induced DNA damage repair.