Toxic component of a type VII toxin-antitoxin (TA) system. A bacteriostatic toxin; upon overexpression in situ or in E.coli inhibits cell growth. Cells swell slightly. Neutralized by cognate antitoxin MntA (PubMed:26112399, PubMed:29555683, PubMed:33045733). Cleaves mRNA in a probably endonucleolytic manner; has no activity on rRNA, tRNA, ssDNA or dsDNA. The TA system confers plasmid stability in E.coli (PubMed:29555683). Neutralization is mostly due to tri-AMPylation by MntA (PubMed:33045733). Forms a complex with cognate antitoxin MntA (PubMed:26112399). Forms a heterooctamer with cognate antitoxin MntA with stoichiometry HepT(6):MntA(2). Two HepT molecules dimerize to form a cleft with 2 Rx4-6H active site motifs in close proximity that probably bind substrate; in the heterooctamer 3 HepT dimers form (PubMed:29555683). Expressed during exponential growth, part of the mntA-hepT operon. Under control of MntA. Tri-AMPylated by cognate antitoxin MntA on Tyr-104; this rotates the residue nearly 180 degrees. Modified protein has much less RNase activity. 6-His tagged, no AMPylation, probably in situ. 6-His tagged, modifed by 3 AMP, probably in situ. When mntA-hepT is deleted, has a sight reduction in swimming motility. No effect when just this gene is deleted. Belongs to the HepT RNase toxin family.