Maturation of precursor 10Sa RNA in Escherichia coli is a two-step process: the first reaction is catalyzed by RNase III in presence of Mn2+.

A precursor to 10Sa RNA accumulates in an rne mutant. However, the present studies indicate that RNase III is the enzyme that processes this RNA. Cell extracts prepared from an rne mutant failed to cleave p10Sa RNA, whereas E coli wild type, rne and rnp cell extracts processed p10Sa RNA under specific assay conditions that require the presence of Mn2+ but not under the customary conditions used for assaying RNase III. That the p10Sa cleaving activity is solely RNase III was confirmed by comparing the increase in p10Sa and poly(A).poly(U) cleaving activities in a strain harboring a plasmid carrying an RNase III gene as compared to a normal E coli strain. It is of interest that these 2 substrates are cleaved by RNase III efficiently, but under 2 different assay conditions. In all strains tested, with normal or elevated levels of RNase III, RNase III fractionates predominantly with the membrane. Further characterization of the maturation of 10Sa RNA revealed that the processing of 10Sa RNA is a 2 step reaction involving 2 separate activities, both sensitive to heat and proteinase K treatment. The first step is catalyzed by RNase III, and results in the formation of a molecule, p10Sa', which is larger than the mature 10Sa RNA. The second activity catalyzes the conversion of p10S' to 10Sa RNA, and this step does not require a divalent cation. The second activity is not any of the known processing endoribonucleases, RNase III, E or P, but could be a new enzyme having no obligate requirement for a divalent cation.