Organization of transfer ribonucleic acid genes in the Escherichia coli chromosome.

The arrangement of transfer ribonucleic acid (RNA) genes in the chromosome of Escherichia coli K-12 (C600) was examined with the techniques of restriction endonuclease digestion and Southern blotting. The number and size of restriction fragments containing transfer or ribosomal RNA sequences or both were estimated by a variety of restriction endonucleases, including EcoRI, BglI, SmaI, SalI, BamHI, and PstI. EcoRI liberated a minimum of 27 fragments which hybridized to transfer RNA and 16 which hybridized to ribosomal RNA. Enzymes which did not cut within the ribosomal RNA operons (PstI and BamHI) liberated 16 and 13 fragments, respectively, which hybridized to transfer RNA. Five PstI and six BamHi fragments also hybridized to ribosomal RNA, suggesting that there may be at least 11 chromosomal locations distinct from ribosomal RNA operons which encode transfer RNA genes. In addition, our data indicated that several transfer RNA genes may be very close to the 5' proximal ends of certain ribosomal RNA operons and close to the 3' distal ends of all seven ribosomal RNA operons. Similar studies have been carried out with 22 purified species of transfer RNA, and we report here the number and size of EcoRI restriction fragments which hybridize to these transfer RNA species.

The distal end of the ribosomal RNA operon rrnD of Escherichia coli contains a tRNA1thr gene, two 5s rRNA genes and a transcription terminator.

The distal region of the Escherichia coli ribosomal RNA operon rrnD has been sequenced to identify transfer RNA genes which might be contained within this transcriptional unit, and to define a possible transcription termination site. A nucleotide sequence corresponding to the gene for tRNA1thr is located between two genes for 5S rRNA. The spacer region between the proximal 5S rRNA gene and the tRNA1thr gene is 12 base pairs long, whereas the spacer region between the distal 5S rRNA gene and the tRNA1thr gene is 37 base pairs. Possible sites for RNA processing are discussed, and the sequence of the tRNA1thr gene is found to possess the same anticodon as the tRNA3thr gene. A putative transcription termination site for rrnD was located about 25 base pairs distal to the 3' end of the second 5S rRNA gene. The rrnD and rrnC terminators are compared with each other and with other prokaryotic terminators. Located at exactly the same site on the complementary strand is another putative transcription termination site for a gene being transcribed towards rrnD. The possibility of overlapping terminators is discussed.