RESUMEN
Comparative in silico analyses of bacterial RNase P enzymes clustered their RNA subunits in type A RNA, found in Escherichia coli, and in type B, found in Bacillus subtilis. Zymomonas mobilis RNase P consists of one protein (Zmo-RnpA) and one type A RNA (RPR) subunit containing the P19 element, present in many RNase P RNAs of any structure class but lacking in the E. coli RNase P RNA. To investigate the putative role of the P19 stem, we constructed a P19 deletion RNA mutant (ΔP19RPR) and performed detailed kinetic analysis of reconstituted enzymes in the presence of the homologous Zmo-RnpA protein or Eco-RnpA protein from E. coli. The deletion of P19 perturbs the monovalent ion requirements. The Mg(2+) requirement for the ΔP19RPR holoenzyme was almost identical to that for the wtRPR holoenzyme at Mg(2+) concentrations of ≤25 mM. Interestingly, enzymes reconstituted with Eco-RnpA protein, relative to those assembled with Zmo-RnpA, exhibited enhanced activity in the presence of ΔP19RPR, suggesting that Eco-RnpA protein can effectively replace its Z. mobilis counterpart. Homologous and heterologous reconstituted enzymes in the presence of ΔP19RPR exhibited differences in their Km values and catalytic efficacies. Overall, the presence of the P19 stem points toward an adaption during the co-evolution of Zmo-RnpA and RPR that is essential for stabilizing the overall structure of the Z. mobilis RNase P. Finally, our results are in line with existing structural data on RNase P enzymes and provide biochemical support for the possible role of appended domains in RNase P RNA subunits.