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Mutants of Escherichia coli K-12 constitutive for fatty acid degradation (fadR) showed an increased rate of utilization of exogenous acetate. Acetate transport, oxidation, and incorporation into macromolecules was approximately fivefold greater in fadR mutants than fadR+ strains during growth on succinate as a carbon source. This effect was due to the elevated levels of glyoxylate shunt enzymes in fadR mutants, since (i) similar results were seen with mutants constitutive for the glyoxylate shunt enzymes (iclR), (ii) induction of the glyoxylate shunt in fadR+ strains by growth on acetate or oleate increased the rate of acetate utilization to levels comparable to those in fadR mutants, and (iii) fadR and fadR+ derivatives of mutants defective for the glyoxylate shunt enzymes showed equivalent rates of acetate utilization under these conditions. These results suggest that the operation of the glyoxylate shunt may play a significant role in the utilization of exogenous acetate by fadR mutants.

The expression of the glyoxylate shunt enzymes is required for growth of Escherichia coli on acetate or fatty acids as a sole carbon source. The genes for the two unique enzymes of the glyoxylate shunt, aceA and aceB, are located at 90 min on the E. coli K-12 genetic map. Polar mutations in the aceB gene eliminate aceA gene function, suggesting that these genes constitute an operon and the direction of transcription is from aceB to aceA. Mu d (Ap lac) fusions with the aceA gene have been constructed to study the regulation of the ace operon. Expression of the ace operon is under the transcriptional control of two genes: the iclR gene, which maps near the ace operon, and the fadR gene, which maps at 25 min, and is also involved in the regulation of the fatty acid degradation (fad) regulon. Merodiploid studies demonstrated that both the iclR and fadR genes regulate the glyoxylate shunt in a trans-dominant manner.

Hybrid derivatives of closely related bacteria may be used to dissect strain-specific functions that contribute to virulence within a host. However, mismatches between DNA sequences are a potent barrier to recombination. Recipients with mutS and recD mutations overcome this barrier, allowing construction of genetic hybrids. To determine whether Salmonella hybrids constructed in a mutS recD host can be used to study virulence, we assayed the effect of mutS and recD mutations on the virulence of Salmonella typhimurium 14028s in mice. Mutants defective in either mutS or recD do not affect the time course or the 50% lethal dose (LD 50 ) of the infection. In contrast, the inactivation of both mutS and recD results in a synthetic phenotype which substantially increases the time required to cause a lethal infection without changing the LD 50 . This phenotype results from an inability of mutS recD double mutants to rapidly adapt to purine-limiting conditions present within macrophages. Although the disease progression is slower, S. typhimurium mutS recD mutants retain the ability to cause lethal infections, and, thus, hybrids constructed in mutS recD hosts may permit the analysis of virulence factors in a surrogate animal model.

The discoveries over the last decade have demonstrated that microbiology is a central scientific discipline with practical applications in agriculture, medicine, bioremediation, biotechnology, engineering, and other fields. It is clear that the roles of microbes in nature are so diverse that the process of mining this genetic variation for new applications will continue long into the future. Moreover, the rapid rate of microbial evolution ensures that there will be no permanent solution to agricultural, medical, or environmental problems caused by microbes. These problems will demand a continual stream of creative new approaches that evolve along with the microbes. Thus, the excitement of this field will continue long into the future. However, these opportunities and imperatives demand a deep understanding of basic microbial physiology, genetics, and ecology. Major challenges that lay ahead are to impart the broad training needed to entice and enable the next generation of microbiologists, and to educate the public and government representatives about the continued and critical importance of this field for health and the economy.

An improved medium for the direct, positive selection of tetracycline-sensitive clones from a population of tetracycline-resistant strains of Escherichia coli is described.

Tandem repeats constitute a consistent fraction of eukaryotic genomes and are given by monomers or gene clusters repeated in a head-to-tail direction. Main examples are represented by microsatellites, minisatellites, satellites, rDNA genes, and protein-coding genes. Tandem repeats dynamics is mainly due to nonreciprocal genomic turnover mechanisms, leading to copy number variation. They differently impact on genome evolution and some of them appear clearly linked to human diseases and are applied for DNA fingerprinting.

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It is now well established that most RNAs are tightly associated with one or more proteins to generate nuclear or cytoplasmic ribonucleoprotein (RNP) complexes. Over 30 years ago, Lerner, Steitz, and coworkers found that the proteic components of these complexes made up some of the antigens recognized by sera of patients affected by systemic lupus erythematosus and other rheumatic diseases. These studies, based on sera derived from several patients, allowed the classification of mammalian small RNPs into several distinct groups. As a result, several RNP complexes composed of a small RNA chain and of one or more proteins have been identified so far. Most of them are localized within nuclei (small nuclear RNPs or snRNPs) and their function, related to nuclear RNA processing, is well characterized. More recently, a minor RNP subset localized in the cytoplasm (small cytoplasmic RNP or scRNP) has also been identified. Structural characterizations have highlighted that different classes of scRNPs exist and are conserved through evolution (they can also be synthesized by viruses). Although the functions of these particles are not completely characterized, different studies suggest their implication in the control of small RNA transcription by RNA polymerase III, protection from nuclease-degradation, and regulation of protein translation.