Archaeal proteins Nop10 and Gar1 increase the catalytic activity of Cbf5 in pseudouridylating tRNA.

Cbf5 is a pseudouridine synthase that usually acts in a guide RNA-dependent manner as part of H/ACA small ribonucleoproteins; however archaeal Cbf5 can also act independently of guide RNA in modifying uridine 55 in tRNA. This guide-independent activity of Cbf5 is enhanced by proteins Nop10 and Gar1 which are also found in H/ACA small ribonucleoproteins. Here, we analyzed the specific contribution of Nop10 and Gar1 for Cbf5-catalyzed pseudouridylation of tRNA. Interestingly, both Nop10 and Gar1 not only increase Cbf5's affinity for tRNA, but they also directly enhance Cbf5's catalytic activity by increasing the k(cat) of the reaction. In contrast to the guide RNA-dependent reaction, Gar1 is not involved in product release after tRNA modification. These results in conjunction with structural information suggest that Nop10 and Gar1 stabilize Cbf5 in its active conformation; we hypothesize that this might also be true for guide-RNA dependent pseudouridine formation by Cbf5.