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The bacterium Escherichia coli O157:H7 is a worldwide threat to public health and has been implicated in many outbreaks of haemorrhagic colitis, some of which included fatalities caused by haemolytic uraemic syndrome. Close to 75,000 cases of O157:H7 infection are now estimated to occur annually in the United States. The severity of disease, the lack of effective treatment and the potential for large-scale outbreaks from contaminated food supplies have propelled intensive research on the pathogenesis and detection of E. coli O157:H7 (ref. 4). Here we have sequenced the genome of E. coli O157:H7 to identify candidate genes responsible for pathogenesis, to develop better methods of strain detection and to advance our understanding of the evolution of E. coli, through comparison with the genome of the non-pathogenic laboratory strain E. coli K-12 (ref. 5). We find that lateral gene transfer is far more extensive than previously anticipated. In fact, 1,387 new genes encoded in strain-specific clusters of diverse sizes were found in O157:H7. These include candidate virulence factors, alternative metabolic capacities, several prophages and other new functions--all of which could be targets for surveillance.

Despite the prokaryotic origins of chloroplasts, a plant chloroplast tRNA precursor is processed in a homologous in vitro system by a pathway distinct from that observed in Escherichia coli, but identical to that utilized for maturation of nuclear pre-tRNAs. The mature tRNA 5' terminus is generated by the site-specific endonucleolytic cleavage of an RNase P (or P-type) activity. The 3' end is likewise produced by a single precise endonucleolytic cut at the 3' terminus of the encoded tRNA domain. This is the first complete structural characterization of an organellar tRNA processing system using a homologous substrate. In contrast to eubacterial RNase P, chloroplast RNase P does not appear to contain an RNA subunit. The chloroplast activity bands with bulk protein at 1.28 g/ml in CsCI density gradients, whereas E.coli RNase P bands as ribonucleoprotein at 1.73 g/ml. Chloroplast RNase P activity survives treatment with micrococcal nuclease (MN) at levels 10- to 100-fold higher than those required to totally inactivate the E.coli enzyme. The chloroplast system is sensitive to a suppression of tRNA processing, caused by binding of inactive MN to pre-tRNA substrate, which is readily overcome by addition of carrier RNA to the assay.