The rate of synthesis of the beta and beta' subunits of RNA polymerase relative to the rate of synthesis of total protein was found to remain constant with increasing steady state growth rate. This is in contrast to the relative synthesis rates of ribosomal proteins which are known to increase with growth rate. Yet the ratio of the rate of transcription of the ribosomal protein (rplJL) and RNA polymerase (rpoBC) domains of the rplKAJLrpoBC gene cluster was found to be invariant. Fusions to lacZ were used to relate the rate of transcription of the rplKAJL genes to the rate of synthesis of total protein. No change was seen at growth rates above 0.8 doublings per hour. This indicates that the growth rate-dependent expression of these ribosomal proteins is regulated at the post-transcriptional level. However because both the relative rate of transcription of rpoBC and rate of synthesis of beta and beta' were found to remain invariant over this growth range it suggests the expression of these RNA polymerase subunits is regulated at the transcriptional level.

The frequency of transcription of the ribosomal protein and RNA polymerase gene segments of the rpJKAJL-rpoBC gene cluster was measured for Escherichia coli K-12 strains carrying mutations in the genes for transcriptional termi,nation factors. The results of our study suggest that Rho increases and that both NusA and the product of sfrB decrease termination frequency in the rplL-rpoB intercistronic region.The concentration of RNA polymerase core enzyme in growing Escherichia coli is determined by the rate of synthesis of the P and ' polypeptides (8,15). The genes that encode these polymerase subunits (rpoBC) are the distal elements in' a gene cluster that also contains four ribosomal protein genes (rplKAJL) (1,10,14,17,18,23,27,31,32). Two major promoters, rplKp and rplJp, are present in this gene cluster ( Fig. 1) (14,27,32). Hience, the rpoBC genes are cotranscribed with at least the rplJL genes. Moreover, it has been reported that many transcripts initiated at rplKp do not terminate after rplA, but continue through rplJp, thus suggesting that all six genes may be cotranscribed (6).Even though the RNA polymerase subunit genes are cotranscribed with upstream ribosomal protein genes, their frequency of transcription is about 20% of that of the upstream genes (7). 'This difference is accounted for by a transcriptional attenuator (atn) in the rplL-rpoB intercistronic region which terminates approximately 80% of the transcripts during steady-state growth (2, 27). DNA sequence analysis has suggested that there is a sequence characteristic of a transcriptional terminator at 43 to 73 base pairs (bp) beyond the end of rplL (22,25). The proposal that this indeed functions as a terminator was further strengthened by Si nuclease and RNA sequence analyses of an in vivo transcript whose 3' end mapped 69 bp beyond rplL (3). The RNA transcript from this region can form an 11-bp stem followed by five consecutive uridine residues (see Fig. 3). This is the canonical structure of a simple or factorindependent terminator. However,'20% of the transcripts continue through this sequence, and various experiments have suggested that the termination frequency may be altered under certain conditions (5,19,20). These results suggest that the function of this sequence may be more complicated than that of a simple constitutive terminator.A number of factors in addition to the RNA polymerase core enzyme and the RNA sequence have been demonstrated to participate in transcriptional termination. The best studied of these is the Rho protein (28). A general class of terminators, referred to as Rho-dependent terminators, has been recognized (13,29), but no simple consensus sequence is evident. What appear to be required are both a nucleotide * Corresponding author. sequence which causes RNA polymerase to pause and an adjacent stretch of RNA devoid of secondary structures (29). Other factors involved in regulating termination are the products of the nusA, nusB, and nusE genes. These were first identified as host genes necessary for N-mediated antit...

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The pattern of transcription of the rplKAJLrpoBC gene cluster of Escherichia coli appears to be complex. At least four different promoters and a transcriptional attenuator have been identified. To compare the relative effect of each of the putative promoters and the attenuator on transcription of these genes, we fused these regulatory sites to lacZ. These transcriptional fusions were constructed on lambda transducing phages so a single copy of each could be stably integrated into the chromosome. The level of ,B-galactosidase in a lysogen of each phage reflects the activity of the transcriptional regulatory site. We find that the promoters preceding rplK (rplKp) and rplJ (rplJp) are indeed the major promoters of this gene cluster. The minor promoter before rplL (rplLp) is much weaker and contributes little to the transcription of the downstream genes. Under these conditions, we find no evidence of a promoter (rpoBp) in the rplL-rpoB intercistronic region. The attenuator (atn) terminates ca. 70o of the transcripts initiated at the promoters preceding it. Although we cannot rule out that some transcripts from rplKp may read through into rplJLrpoBC, we find that rplJp alone is sufficient for high-level expression of these genes.The genes for the subunits of Escherichia coli RNA polymerase are cotranscribed with ribosomal protein genes. The genes for ,B and 13' (rpoB and rpoC) are cotranscribed with at least the L10 and L7/12 ribosomal protein genes (rplL and rplJ, respectively) (26,33,42), whereas the a gene (rpoA) is part of an operon containing four ribosomal protein genes (23). More recently, it has been shown that the gene for a (rpoD) is cotranscribed with the S21 gene (rpsU) and the gene for DNA primase (dnaG) (10).The transcription pattern of the rplKAJLrpoBC gene cluster located at 90 min on the E. coli chromosome is complicated. Initial experiments demonstrated that the genes for the 1B and 1' subunits of RNA polymerase are cotranscribed with at least the rplJ and rplL ribosomal protein genes from a promoter upstream of rplJ (rpUp) (26,33,42). A variety of experiments subsequently suggested that there are also two internal promoters within rplJLrpoBC, one preceding rplL (rplLp) and the other before rpoB (rpoBp) (20,25,26,28,33). In addition, a transcriptional attenuator (atn) was located between rplL and rpoB and accounts for the lower frequency of rpoBC transcription compared with that of rplJL (5,6,14,26). A promoter responsible for the cotranscription of rplK and rplA was identified upstream of rplK (rplKp) (2,15,26,43). Recent S1 nuclease mapping of in vivo transcripts suggests that transcription initiated at rplKp is not necessarily terminated after rplA, but can read through into the rplJL genes (9).To further understand the regulation of this complex operon, we have directly compared the relative effect of these different promoters and the attenuator on rplKAJLrpoBC transcription. These transcriptional regulatory sites were fused, both separately and in combination, to the lacZ gene carried on a lambd...