Electron microscope heteroduplex studies of sequence relations among plasmids of Escherichia coli: VII. Mapping the ribosomal RNA genes of plasmid F14

Abstract

Escherichia coli episome F14 contains two 16 S + 23 S ribosomal RNA gene sets. Their positions and structure have been studied by electron microscope heteroduplex methods. The A gene set maps from 81.4 to 86.7 kilobases from the counterclockwise junction of F DNA with bacterial chromosomal DNA of F14; the B gene set maps very close to the clockwise junction, that is, at map position 204.4 to 209.7 kilobases from the counterclockwise junction. The structure of the rRNA-rDNA hybrids indicates that each gene set consists of one 16 S gene (r16) of length 1.28 ± 0.12 kilobases, a spacer (rsp) of length 0.57 ± 0.12 kilobase and a 23 S gene (r23) of length 2.60 ± 0.20 kilobases. We propose the genetic notation rrn for the entire gene set. The structure of the DNA/DNA duplexes between the rrnA set and the rrnB set shows that there is a region of non-homology of length 0.29 ± 0.06 kilobase within the otherwise homologous rrnA and rrnB sequences. This non-homology region is probably part, but not all, of the spacer between the 16 S and 23 S genes.