Escherichia coli ribosomal ribonucleic acids are not cut from an intact precursor molecule.

Ribonuclease III-deficient strains of Escherichia coli accumulate a "30 S" RNA species. Kinetic analysis of label incorporated into rRNA species shows, however, that 30 S RNA is not the major precursor to the 16 S and 23 S RNA species is in 7- to 10-fold excess over that into 30 S RNA. The 30 S RNA species turns over with a half-life of about 2.5 min, which could account for no more than one-tenth of the incorporation into 16 S plus 23 S RNA. Thus, under these conditions RNase III-strains of E. coli do not cut rRNAs from an intact tandem precursor molecule but rather from the elongating nascent transcript, possibly by an alternate pathway not involving RNase III.