Role of precursor sequences in the ordered maturation of E. coli 23S ribosomal RNA.

The maturation of ribosomal RNAs (rRNAs) is an important but incompletely understood process required for rRNAs to become functional. In order to determine the enzymes responsible for initiating 3' end maturation of 23S rRNA in Escherichia coli, we analyzed a number of strains lacking different combinations of 3' to 5' exo-RNases. Through these analyses, we identified RNase PH as a key effector of 3' end maturation. Further analysis of the processing reaction revealed that the 23S rRNA precursor contains a CC dinucleotide sequence that prevents maturation from being performed by RNase T instead. Mutation of this dinucleotide resulted in a growth defect, suggesting a strategic significance for this RNase T stalling sequence to prevent premature processing by RNase T. To further explore the roles of RNase PH and RNase T in RNA processing, we identified a subset of transfer RNAs (tRNAs) that contain an RNase T stall sequence, and showed that RNase PH activity is particularly important to process these tRNAs. Overall, the results obtained point to a key role of RNase PH in 23S rRNA processing and to an interplay between this enzyme and RNase T in the processing of different species of RNA molecules in the cell.