<div><p>RNase III–related enzymes play key roles in cleaving double-stranded RNA in many biological systems. Among the best-known are RNase III itself, involved in ribosomal RNA maturation and mRNA turnover in bacteria, and Drosha and Dicer, which play critical roles in the production of micro (mi)–RNAs and small interfering (si)–RNAs in eukaryotes. Although RNase III has important cellular functions in bacteria, its gene is generally not essential, with the remarkable exception of that of <em>Bacillus subtilis</em>. Here we show that the essential role of RNase III in this organism is to protect it from the expression of toxin genes borne by two prophages, Skin and SPβ, through antisense RNA. Thus, while a growing number of organisms that use RNase III or its homologs as part of a viral defense mechanism, <em>B. subtilis</em> requires RNase III for viral accommodation to the point where the presence of the enzyme is essential for cell survival. We identify <em>txpA</em> and <em>yonT</em> as the two toxin-encoding mRNAs of Skin and SPβ that are sensitive to RNase III. We further explore the mechanism of RNase III–mediated decay of the <em>txpA</em> mRNA when paired to its antisense RNA RatA, both <em>in vivo</em> and <em>in vitro</em>.</p> </div
