The functional exchangeability of pk- and k-turns in RNA structure

Abstract

Ribonuclease p RNA requires a sharply kinked RNA helix to make a loop-receptor interaction that creates the binding site for the substrate. In some forms of the ribozyme, this is accomplished by a k-turn, while others have a different element called the pk-turn. The structure of the pk-turn in RNase p of Thermotoga maritima is globally very similar to a k-turn, but lacks all the standard features of that structure, including long-range hydrogen bonds between the two helical arms. We show here that in an isolated RNA duplex, the pk-turn fails to adopt a tightly kinked structure, but rather is a flexible element. This suggests that the tertiary contacts of RNase p assist its folding into the required kinked structure. We find that we can replace the k-turn of the sAM-I riboswitch with the pk-turn, such that the resulting RNA retains its ability to bind sAM, although with lower affinity. We also find that we can replace the pk-turn of T. maritima RNase p with a standard k-turn (in either orientation) with retention of ribozyme activity. Thus, although the pk-turn cannot intrinsically fold into the kinked structure, it can be induced to fold correctly in context. And the pk-turn and k-turns can substitute functionally for one another. © 2013 Landes Bioscience.