Antitoxin component of a potential type II toxin-antitoxin (TA) system. Acts as an antitoxin against host toxins RnlA and LsoA, preventing them from degrading T4 bacteriophage-derived mRNA and thus permitting successful virus infection. Stabilizes middle (8-10 minutes) and late (18 to 28 minutes) T4 gene transcripts. Can form a complex with non-cognate host toxins LsoA and RnlA. Phage grows very poorly in wild-type E.coli K12 or in cells expressing plasmid-derived toxin-antitoxin system LsoA-LsoB; if the host has a deletion in rnlA-rnlB no effect is seen. Absence of full-length transcripts of late (18 to 28 minutes) T4 genes, and thus loss of synthesis of late proteins. Partially suppressed by overexpression of host IscR. Enterobacteria phage T4 probably has no corresponding toxin gene.