Figure 1.

Schematic representation of the Rtc system

(A) The rtcBA operon is under the control of the sigma 54 dependent rtcBA promoter whose activator is encoded by the oppositely oriented rtcR gene. RtcA is an RNA 3′-terminal phosphate cyclase that converts 2′-P and 3′-P ends into 2′,3′-cyclic P ends. RtcB is an RNA ligase that ligates 3′-P and 2′,3′-cyclic P ends to 5′-OH ends.

(B) The sigma 54 transcriptional activator RtcR has three functional domains including a N-terminal, regulatory CARF domain. Under normal conditions, the CARF domain represses the AAA + domain so that no transcription of rtcBA takes place from the rtcBA promoter (left). Conversely, RtcR mutants lacking the CARF domain are constitutively active (middle). Under stress, an unknown signal ligand interferes with the CARF domain mediated repression of the AAA + domain and the rtcBA operon is expressed (right).