Cell 50: [495][496][497][498][499][500][501][502][503][504][505][506][507][508] 1987), multiple copies of chromosomal sequence are found at 68 and at 64 to 65 min (M. Umeda and E. Ohtsubo, J. Mol. Biol. 213:229-237, 1990). We have determined that the rnpB gene (previously mapped at 70 min [B. J. Bachmann, Microbiol. Rev. 54:130-197, 1990]) is located within these segments of repeated sequences as five separate copies, together with tdcA, B, C, and R (mapped at 68 min [Bachmann, 1990]) and six unidentified open reading frames. Since close linkage of rnpB and tdc is found in various strains of E. coli K-12, the rnpB gene should be mapped at 68 min rather than 70 min.In Kohara's library derived from Escherichia coli K-12 W3110 (6), a chromosomal duplication which is assumed to be mediated by IS5 has been identified: at 64 to 65 min, a 12.5-kb segment flanked by IS5 is tandemly repeated three times, and at 68 min, the same segment is repeated 1.7 times (15) (Fig. 1A). Comparisons of the restriction maps of these regions in W3110 and those in other strains revealed that this duplication is characteristic of W3110 and that the multiple copies are derived from a segment at 68 min (3,15).Prior to the present study, we made an exhaustive survey of tRNA genes in E. coli by analyzing the tRNAs produced upon infection of each lambda clone in Kohara's library ("miniset," 476 clones) (7). In addition to tRNAs, some small stable RNAs were detected, and these RNAs included some unidentified RNAs. Clones 479, 480, 501, and 502 (at 64 to 65 min) and 514, 515, and 516 (at 68 min), which correspond to the multicopy regions mentioned above, produced stable RNAs of about 400 nucleotides ( Fig. 2) which we have now identified.A candidate for these RNAs was Ml RNA (the RNA component of RNase P), even though the rnpB gene which encodes Ml RNA has been mapped at 70 min (2). We examined the complementation activity of these phage clones in E. coli ts709, an rnpB temperature-sensitive mutant (11). Lambda bacteriophages 479, 480, 501, 502, 514, 515, and 516 were cross-streaked against strain ts709, which had been lysogenized with wild-type phage to provide products of the cI gene, and incubated at the restrictive temperature (42°C). All of the seven X clones were able to complement the rnpB709(Ts) mutation (data not shown). On the other hand, no phage clone corresponding to the 70-min region produced any stable RNA and complemented the rnpB709(Ts) mutation (data not shown). It therefore seemed likely that the rnpB gene is located at 68 min rather than at 70 min, and in strain W3110, the segment containing the rnpB gene was repeated, conserving functional genes.To determine the exact location of the rnpB gene, we constructed plasmids that carried various restriction fragments of phage 515 DNA, which corresponds to the region presumed to be the original segment of the chromosome. Among these plasmids, one carrying an EcoRV 4.4-kb fragment appeared to complement the rnpB709(Ts) mutation * Corresponding author.(data not shown). Nucleotide seque...