Structure and mechanistic features of the prokaryotic minimal RNase P

Endonucleolytic removal of 59-leader sequences from tRNA precursor transcripts (pre-tRNAs) by RNase P is essential for protein synthesis. Beyond RNA-based RNase P enzymes, protein-only versions of the enzyme exert this function in various Eukarya (there termed PRORPs) and in some bacteria (Aquifex aeolicus and close relatives); both enzyme types belong to distinct subgroups of the PIN domain metallonuclease superfamily. Homologs of Aquifex RNase P (HARPs) are also expressed in some other bacteria and many archaea, where they coexist with RNA-based RNase P and do not represent the main RNase P activity. Here we solved the structure of the bacterial HARP from Halorhodospira halophila by cryo-EM revealing a novel screw-like dodecameric assembly. Biochemical experiments demonstrate that oligomerization is required for RNase P activity of HARPs. We propose that the tRNA substrate binds to an extended spike-helix (SH) domain that protrudes from the screw-like assembly to position the 59-end in close proximity to the active site of the neighboring dimer subunit. The structure suggests that eukaryotic PRORPs and prokaryotic HARPs recognize the same structural elements of pre-tRNAs (tRNA elbow region and cleavage site). Our analysis thus delivers the structural and mechanistic basis for pre-tRNA processing by the prokaryotic HARP system.