Toxic component of a type II toxin-antitoxin (TA) system. Upon overexpression causes bacteriostasis. Its effect is neutralized by coexpression with cognate antitoxin PhD. May bind the 30S ribosomal subunit. In M.smegmatis 3 TA systems (VapB-VapC, MazE-MazF and Phd-Doc) may be involved in monitoring the nutritional supply and physiological state of the cell, linking catabolic with anabolic reactions. Constitutively expressed in all growth phases, part of the MSMEG_1276-phd-doc-MSMEG_1279 operon. Negatively autoregulated by one or both of Phd-Doc. The phd-doc operon is not essential. A triple TA mutant (missing vapB-vapC, mazE-mazF and phd-doc TA systems) survives antibiotic challenge, suggesting the TA systems are not required to generate drug-resistant cells. However the mutant is more sensitive to oxidative and heat stress, and does not survive long term starvation during aerobic growth on complex medium. There is a difference in the level of branched-chain amino acids, which may play a role in monitoring the nutritional supply and physiological state of the cell.