Increased rrn gene dosage causes intermittent transcription of rRNA in Escherichia coli.

When the number of rRNA (rrn) operons in an Escherichia coli cells is increased by adding an rrn operon on a multicopy plasmid, the rate of rRNA expression per operon is reduced to maintain a constant concentration of rRNA in the cell. We have used electron microscopy to examine rRNA transcription in cells containing a multicopy plasmid carrying rrnB. We found that there were fewer RNA polymerase molecules transcribing the rrn genes, as predicted from previous gene dosage studies. Furthermore, RNA polymerase molecules were arranged in irregularly spaced groups along the operon. No apparent pause or transcription termination sites that would account for the irregular spacing of the groups of polymerase molecules were observed. We also found that the overall transcription elongation rate was unchanged when the rrn gene dosage was increased. Our data suggest that when rrn gene dosage is increased, initiation events, or promoter-proximal elongation events, are interrupted at irregular time intervals.