Toxic component of an atypical, type II toxin-antitoxin chaperone (TAC) system. Upon expression in M.smegmatis inhibits colony formation and cell growth (PubMed:20011113, PubMed:23927792). Ectopic expression in wild-type M.tuberculosis has no effect on cell growth; ectopic expression in a triple higB1-higA1-Rv1957 (delta TAC) disruption mutant causes growth arrest, killing a considerable proportion of the cells. Increased ectopic expression leads to decreased levels of IdeR- and Zur-regulated genes as well as cleavage within the mRNA region of tmRNA (transfer-mRNA), strongly suggesting it is an endoribonuclease; also degrades E.coli and M-smegmatis tmRNA (PubMed:23927792). Its toxic effect is neutralized by coexpression with antitoxin HigA. Neutralization of HigB1 toxin in E.coli or M.marinum also requires SecB-like chaperone Rv1957, making this the first toxin-antitoxin chaperone (TAC) system (PubMed:21536872). Induced by hypoxia and by DNA damaging agent mitomycin C. Part of the Rv1954A-higB1-higA1-Rv1957 operon, as well as the higB1-higA1-Rv1957 operon, which is probably the mitomycin-induced operon; the former but not latter operon is autorepressed by HigA1 (PubMed:20585061). Induced in persister cells in response to D-cycloserine (PubMed:21673191). A triple higB1-higA1-Rv1957 disruption mutant has no visible phenotype. Belongs to the mycobacterial HigB family. Upon expression in E.coli, Rv1955 has been shown to function as an antitoxin against Rv1956 (PubMed:19016878). It is not clear if these conflicting results are due to expression in a heterologous system. The gene names higA and higB have been assigned to both Rv1955 and Rv1956; we have chosen to call Rv1955 higB1 after consulting the authors.