FIGURE 1.

Bacterial rtcBA operons and pathway options for RtcA in RNA repair. (A) The RNA ligase RtcB is encoded in a two-gene operon in E. coli along with the RNA cyclase RtcA. (B) The cyclic phosphodiesterase activity of RtcB hydrolyzes a 2′,3′-cyclic phosphate to a 3′-phosphate. The ligase activity of RtcB then joins the RNA 3′-phosphate to a 5′-OH RNA end to yield a 3′-5′ phosphodiester splice junction. RtcA converts RNA 3′-phosphate ends to 2′,3′-cyclic phosphates, a reaction that appears wasteful from the perspective of RtcB. (C) Were RtcA able to cyclize RNA 2′-phosphate ends, which are not substrates for sealing by RtcB, it might serve a useful function in an RNA repair pathway, as shown. (D) Schematic diagram of the Salmonella typhimurium operon in which rtcB and rtcA are preceded by a co-oriented ORF encoding Ro and followed by a co-oriented toxin-antitoxin cassette encoding the mRNA ribotoxin YafQ and its antitoxin DinJ. The operon is regulated by RtcR, a σ⁵⁴ coactivator encoded by an adjacent, oppositely oriented gene. Exemplary bacterial taxa that have an RtcR-regulated ro-RtcB-RtcA operon are listed; the sizes (aa) of the component proteins are shown.