Toxic component of a type II toxin-antitoxin (TA) system. A probable translation-independent mRNA interferase. Overexpression causes cessation of cell growth and inhibits cell proliferation via inhibition of translation; this blockage is overcome (after 90 minutes) by subsequent expression of antitoxin HicB. Overexpression causes cleavage of a number of mRNAs and tmRNA, in a translation-independent fashion, suggesting this is an mRNA interferase (PubMed:19060138). Probably forms a complex with the antitoxin HicB which inhibits the mRNA interferase activity. Induced by amino acid starvation, carbon starvation and when translation is blocked. Induction no longer occurs in the absence of Lon protease suggesting, by homology to other toxin-antitoxin systems, that it may degrade the HicB antitoxin. A member of the hicA-hicB operon. Belongs to the HicA mRNA interferase family.