ABSTRACT
RtcB is a noncanonical RNA ligase that joins either 2',3'-cyclic phosphate or 3'-phosphate termini to 5'-hydroxyl termini. The genes encoding RtcB and Archease constitute a tRNA splicing operon in many organisms. Archease is a cofactor of RtcB that accelerates RNA ligation and alters the NTP specificity of the ligase from Pyrococcus horikoshii. Yet, not all organisms that encode RtcB also encode Archease. Here we sought to understand the differences between Archease-dependent and Archease-independent RtcBs so as to illuminate the evolution of Archease and its function. We report on the Archease-dependent RtcB from Thermus thermophilus and the Archease-independent RtcB from Thermobifida fusca. We find that RtcB from T. thermophilus can catalyze multiple turnovers only in the presence of Archease. Remarkably, Archease from P. horikoshii can activate T. thermophilus RtcB, despite low sequence identity between the Archeases from these two organisms. In contrast, RtcB from T. fusca is a single-turnover enzyme that is unable to be converted into a multiple-turnover ligase by Archease from either P. horikoshii or T. thermophilus. Thus, our data indicate that Archease likely evolved to support multiple-turnover activity of RtcB and that coevolution of the two proteins is necessary for a functional interaction.