Functional Characterization of Sirtuin-like Protein in Mycobacterium smegmatis

Nicotinamide adenine dinucleotide (NAD)-dependent deacetylases (sirtuins) are well conserved from prokaryotes to eukaryotes. Functions and regulations of mammalian sirtuins have been extensively studied and indicate that sirtuins play an important role in regulation of biological processes, whereas functions of mycobacterial sirtuins were less explored. To examine functions of the sirtuin-like protein in mycobacteria, a Mycobacterium smegmatis sirtuin, MSMEG_5175, was overexpressed in a M. smegmatis strain mc(2)155 to generate an MSMEG_5175-overexpression strain (mc(2)155-MS5175) in the present study. The physiological aspects of mc(2)155-MS5175 strain were characterized showing that they had a lower intracellular NAD level and a higher resistance to isoniazid (INH) as compared to mc(2)155 containing empty pMV261 plasmid (mc(2)155-pMV261). Quantitative proteomic analysis was carried out to determine differentially expressed proteins between mc(2)155-pMV261 and mc(2)155-MS5175. Among 3032 identified proteins, overexpression of MSMEG_5175 results in up-regulation of 34 proteins and down-regulation of 72 proteins, which involve in diverse cellular processes including metabolic activation, transcription and translation, antioxidant, and DNA repair. Down-regulation of catalase peroxidase (KatG) expression in both mRNA and protein levels were observed in mc(2)155-MS5175 strain, suggesting that a decrease in cellular NAD content and down-regulation of KatG expression contribute to the higher resistance to INH in mc(2)155-MS5175. Using a combination of immunoprecipitation and proteomic analysis, we found that acetylation in 27 proteins was decreased in mc(2)155-MS5175 as compared to those in mc(2)155-pMV261, suggesting that these proteins including the beta prime subunit of RNA polymerase (rpoC), ribosomal proteins, and metabolic enzymes were substrates of MSMEG_5175. Acetylation changes in rpoC may affect its function and cause changes in global gene transcription. Taken together, these results suggest that MSMEG_5175 regulates diverse cellular processes resulting in an increase in INH resistance in mycobacteria, and provide a useful resource to further biological exploration into functions of protein acetylation in mycobacteria.