A structural and functional study of plastid RNAs homologous to catalytic bacterial RNase P RNA.

Ribonuclease P (RNase P), the ubiquitous enzyme required for 5' maturation of transfer RNA, is a ribonucleoprotein containing an essential RNA subunit. This RNA (P RNA) is the catalytic component of RNase P in Bacteria and some Archaea. A putative P RNA is encoded in the chloroplast genome of three algae: Cyanophora paradoxa, Porphyra purpurea and Nephroselmis olivacea. In no case, the P RNAs from the plastids were active in vitro in conditions where bacterial and some archaeal P RNAs are functional. By using lead-ion-induced hydrolysis, we conclude that the catalytic deficiency is most likely due to the perturbation of the global structure of the plastid P RNAs compared to the bacterial counterpart. As a consequence, the plastid P RNAs are unable to bind to the precursor tRNA substrates. We discuss these results in the context of plastid and RNase P evolution.