Here we present evidence that only five of the seven rRNA operons present in Escherichia coli are necessary to support near-optimal growth on complex media. Seven rrn operons are necessary, however, for rapid adaptation to nutrient and temperature changes, suggesting it is the ability to adapt quickly to changing environmental conditions that has provided the selective pressure for the persistence of seven rrn operons in E. coli. We have also found that one consequence of rrn operon inactivation is a miscoordination of the concentrations of initiation factor IF3 and ribosomes.In Escherichia coli, the number of ribosomes per cell varies in proportion to the growth rate to meet the cell's changing demand for protein synthesis. At doubling times of around 20 min, there are as many as 70,000 ribosomes per E. coli cell, while at lower growth rates there may be only 20,000 ribosomes per cell (5). This control of ribosome content is exerted at the level of transcription of the seven rRNA operons (rrn) present on the E. coli genome (16,22,29). It is generally assumed that this redundancy of rRNA operons in E. coli exists to support the high levels of ribosome production necessary for rapid growth rates (21,27).Many other bacteria also possess multiple rRNA transcription units. In Bacillus subtilis (3,23) and Clostridium perfringens (14), there are 10 rrn operons, and in Lactococcus lactis, there are 6 (1, 32). There are also examples of organisms with few rrn operons: Mycoplasma capricolum has two (30), and the archaebacterium Halobacterium cutirubrum possesses only one (20). In general, organisms with multiple rrn operons are capable of achieving faster doubling times than those with just one or two. In both E. coli (9, 11) and B. subtilis (34), it has been demonstrated that one operon can be deleted without a major influence on cell growth rate or physiology. Furthermore, we have previously obtained evidence suggesting that, even at optimal growth rates, the seven E. coli rrn operons do not function at their maximal potential but are capable of higher levels of expression under certain conditions (8). These observations suggest that neither organism should really require its full complement of rrn operons to sustain high growth rates, and yet the persistence of 7 or 10 functional operons on their genomes suggests some selective advantage to the organism of retaining all of its rrn genes.The seven rrn operons in E. coli are located in several noncontiguous sites on one-half of the chromosome, centered around the origin of replication, oriC (12). Like most other highly expressed genes, the rrn operons are all transcribed in the same direction in which the chromosome is replicated, which serves to prevent collisions between RNA and DNA polymerases (13). All seven operons have approximately the same organization: tandem promoters, P1 and P2-16S-spacer tRNA(s)-23S-5S-distal tRNA(s) (6, 21). Four of the operons contain tRNAGlu-2 in their spacer regions, and three contain tRNA Ala-1B plus tRNA , while at the 3Ј end of the o...

Borrelia burgdorferi spends a significant proportion of its life cycle within an ixodid tick, which has a cuticle containing chitin, a polymer of N-acetylglucosamine (GlcNAc). The B. burgdorferi celA, celB, and celC genes encode products homologous to transporters for cellobiose and chitobiose (the dimer subunit of chitin) in other bacteria, which could be useful for bacterial nutrient acquisition during growth within ticks. We found that chitobiose efficiently substituted for GlcNAc during bacterial growth in culture medium. We inactivated the celB gene, which encodes the putative membrane-spanning component of the transporter, and compared growth of the mutant in various media to that of its isogenic parent. The mutant was no longer able to utilize chitobiose, while neither the mutant nor the wild type can utilize cellobiose. We propose renaming the three genes chbA, chbB, and chbC, since they probably encode a chitobiose transporter. We also found that the chbC gene was regulated in response to growth temperature and during growth in medium lacking GlcNAc.

We identified a large outbreak of rapidly growing mycobacterial infections among persons who had had footbaths and pedicures at one nail salon. Physicians should suspect this cause in patients with persistent furunculosis after exposure to whirlpool footbaths.

The covalent cross-linking of unmodified Escherichia coli N-acetylvalyl-tRNA to the 16S RNA of Escherichia coli ribosomes upon near-UV irradiation previously reported by us [Schwartz, I., & Ofengand, J. (1978) Biochemistry 17, 2524--2530] has been studied further. Up to 70% of the unmodified tRNA, nonenzymatically bound to tight-couple ribosomes at 7 mM Mg2+, could be cross-linked by 310--335-nm light. Covalent attachment was solely to the 16S RNA. It was dependent upon both irradiation and the presence of mRNA but was unaffected by the presence or absence of 4-thiouridine in the tRNA. The kinetics of cross-linking showed single-hit behavior. Twofold more cross-linking was obtained w-th tight-couple ribosomes than with salt-washed particles. Puromycin treatment after irradiation released the bound N-acetyl[3H]valine, demonstrating that the tRNA was covalently bound at the P site and that irradiation and covalent linking did not affect the peptidyl transferase reaction. Cross-linking was unaffected by the presence of O2, argon, ascorbate (1 mM), or mercaptoethanol (10 mM). Prephotolysis of a mixture of tRNA and ribosomes in the absence of puly(U2,G) did not block subsequent cross-linking in its presence nor did it generate any long-lived chemically reactive species. There was a strong tRNA specificity. E. coli tRNA1Val and tRNA1Ser and Bacillus subtilis tRNAVal and tRNAThr could be cross-linked, but E. coli tRNA2Val, 5-fluorouracil-substituted tRNA1Val, tRNAPhe, or tRNAFMet could not. By sequence comparison of the reactive and nonreactive tRNAs, the site of attachment in the tRNA was deduced to be the 5'-anticodon base, cmo5U, or ,o5U in all of the reactive tRNAs. The attachment site in 16S RNA is described in the accompanying paper [Zimmerman, R. A., Gates, S. M., Schwartz, I., & Ofengand, J. (1979) Biochemistry (following paper in this issue)]. The link between tRNA and 16S RNA is either direct or involves mRNA bases at most two nucleotides apart since use of the trinucleotide GpUpU in place of poly(U2,G) to direct the binding and cross-linking of N-acetylvalyl-tRNA to the P site did not affect either the rate or yield of cross-linking. Both B. subtilis tRNAVal (mo5U) and E. coli tRNA1Val (cmo5U) gave the same rate and yield of cross-linking when directed by the trinucleotide GpUpU. Therefore, the presence of the charged carboxyl group in the cmo5U-containing tRNA apparently does not markedly perturb the orientation of this base with respect to its reaction partner in the 16S RNA. The cross-linking of AcVal-tRNA only takes place from the P site. At 75 mM KCl and 75 mM NH4Cl, less than 0.4% cross-linking was found at the A site, while 55.5% was obtained at the P site. However, when the salt concentration was lowered to 50 mM NH4Cl, 5% cross-linking to the A site was detected, compared to 49% at the P site. Thus, a simple change in the ionic strength of the incubation mixture was able to alter the affinity labeling pattern of the ribosome.

Lineages of Borrelia burgdorferi, the bacterium that causes Lyme disease, can be characterized by distinct alleles at the outer surface protein C (ospC) locus. The lineages marked by ospC genotypes have been shown to be differentially invasive in different species of mammals, including humans; genotypes A, B, I, and K effectively disseminate to human blood and cerebrospinal fluid. In this report, we extend the sample of genotypes isolated from human blood to include genotypes N, H, C, M, and D, and rank each by their probability of disseminating from ticks to the blood of humans. Our results demonstrate that only some genotypes of B. burgdorferi present in ticks have a high propensity to disseminate in humans.

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Eight cases are reported of an unusual breast lesion consisting of dense intralobular, perilobular, and perivascular lymphocytic infiltrates associated with lobular atrophy and sclerosis. Immunologic studies on paraffin-embedded tissue (seven cases) and frozen tissue (two cases) demonstrated a predominance of B-cells and expression of HLA-DR antigen in involved lobular epithelium. The immunologic features of this lesion are reminiscent of lymphoepithelial lesions of salivary glands and other sites. A possible autoimmune pathogenesis is suggested.