The molecular organisation of the Prochlorococcus marinus rnpB gene and the catalytic activity of the encoded RNA were characterised. Kinetic parameters for several pre-tRNA substrates were comparable to those from other eubacterial RNase P RNAs, although unusually high cation concentrations were required. The CCA-end of pre-tRNAs is essential for efficient turnover despite the lack of the canonical binding motif in P. marinus RNase P RNA. A trnR gene is located only 38 nt upstream the rnpB 5' end on the complementary strand. This arrangement resembles those in the plastids of Cyanophora and Porphyra but not in any other bacterium.