Evidence for Co-transcription of the RNA polymerase genes rpoBC with a ribosomal protein gene of Escherichia coli

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Abstract
The adjacent genes rpoB and rpoC code for the β and β′ subunits of RNA polymerase in Escherichia coli, and are cotranscribed in the order given. The nearest known genes to rpoB are rplL and rplA,J,K, which code for ribosomal proteins, and which are transcribed in the same direction as the polymerase genes. It has been suggested that rpoBC may be distal elements of a larger operon including these ribosomal genes. To test this possibility we have cloned a segment of DNA, derived by endoR. HindIII digestion from the rpoBC-transducing bacteriophage λrif d 18, in the replacement vector NMλ761. The structure of the λrpoBC bacteriophages so produced is such that the inserted DNA can be transcribed from λ promoters, allowing us to confirm that it carries intact rplL, rpoB, and rpoC genes. We have studied these bacteriophages as lysogens in rec + and rec bacteria, and by infection of UV-irradiated bacterial strains in which λ promoters are either repressed or active. The results indicate that the cloned DNA contains at most a very weak promoter for the above genes, in contrast to that present in the larger segment of bacterial DNA carried by λrif d 18. We have in the same way cloned the adjacent bacterial HindIII-fragment of λrif d 18 DNA, and have found that it displays vigorous autonomous expression of the tufB, rplA, and rplK genes. We conclude that rpoB and C are obligatorily co-transcribed with rplL, from a promoter located outside the DNA segment cloned in λrpoBC. We discuss the evidence for the existence of a regulatory site, rpoU, located between rplL and rpoB.