Transcription and processing of prokaryotic rRNA:

In E.  coli  there  are  seven  rRNA  transcription  units  scattered  by  the genome,  every of that contains one copy of each of the 16S, 23S  and 5S rRNA genes and one to four copies of several tRNA genes which is shown in the diagram. This gene assembly is transcribed through the single prokaryotic RNA polymerase to yield a single 30S pre-rRNA transcript about 6000 nt in the size. This arrangement ensures that stoichiometric amounts   of   the   various   rRNAs   are   synthesized   for   ribosome assembly.  Subsequently transcription, the 30S pre-rRNA  molecule  forms  internal base  paired  regions  to  provide  a  series  of  stem-loop  structures  and  ribosomal proteins  bind  to form  a ribonucleoprotein (RNP)  complex.  The number of the nucleotides in the folded pre-rRNA molecule is now methylated on the ribose moieties by using S-adenosylmethionine as the methyl donor. Subsequently the pre-rRNA fragment is cleaved at specific sites through RNase III to release precursors of the 23S, 16S and 5S rRNAs. The precursors are then trimmed at their 5' and 3' ends through ribonucleases M23, M5 and M16 that will act on the 5S, 16S and 23S precursor rRNAs respectively to generate the mature rRNAs.

Figure:  Transcription and processing of prokaryotic rRNA.

Figure:  (a) rRNA transcription  units; (b) transcription  of a single transcription unit by RNA Pol I and processing of pre-rRNA.