The number of rRNA genes in Escherichia coli

The three different ribosomal RNA species of Escherichia colt’ (5 S, 16 S and 23 S) originate from a single 30 S precursor RNA. The DNA coding for this 30 S rRNA is called an rRNA gene set (rm). It has long been known that these genes are redundant in bacteria, their number has been estimated on the basis of saturation hybridization experiments as being 510 (for a review see [ 1 ] ). The most reliable measurements suggested the existence of six rm genes [2]. Plasmids and transducing phages helped to map unambiguously five of these, which are now designated rrnA (85 min), rrnB (88 min), rrnC (83 min), rmD (71 min) and rmE (89 min) [3]. In this paper we present evidence strongly suggesting that the actual number of rRNA gene sets is seven in E. coli K12.