this diversity is located in discrete gene clusters that are spread throughout the different genomes. In contrast to this diversity, these enteric microorganisms exhibit marked synteny in their largescale genomic organization, bearing in mind that E. coli and S. enterica diverged about 100 Myr ago 28. The conserved genes may be a re¯ection of the basic lifestyle of the bacteria, requiring intestine colonization, environmental survival and transmission. The unique gene clusters probably contribute to adaptation to environmental niches and to pathogenicity. The pseudogene complement of S. typhi has implications for our understanding of the tight host restriction of this organism, and raises the question of whether it may be possible to eradicate S. typhi and typhoid fever altogether. M Methods Salmonella typhi CT18 was isolated in December 1993, at the Mekong Delta region of Vietnam, from a 9-year-old girl who was suffering from typhoid. The strain was isolated from blood using routine culture methods 23 , and after serological and metabolic con-®rmation of the strain as S. typhi it was immediately frozen in glycerol at-70 8C. The genome sequence was obtained from 97,000 end sequences (giving 7.9´coverage) derived from several pUC18 genomic shotgun libraries (with insert sizes ranging from 1.4 to 4.0 kb) using dye terminator chemistry on ABI377 automated sequencers. This was supplemented with 0.7´sequence coverage from M13mp18 libraries with similar insert sizes. End sequences from a larger insert plasmid (pSP64; 1.9´clone coverage, 10±14-kb insert size) and lambda (lambda-FIX-II; 0.4´clone coverage, 20±22-kb insert size) libraries were used as a scaffold, and the ®nal assembly was veri®ed by comparison with restriction-enzyme digest patterns using pulsed-®eld gel electrophoresis (data not shown). Total sequence coverage was 9.1´. The sequence was assembled, ®nished and annotated as described 29 , using Artemis 30 to collate data and facilitate annotation. In addition we used a gene®nder that was trained speci®cally for S. typhi, which uses a hidden Markov model with modules for the coding region, start and stop codons, and the ribosome-binding site (T.S.L. and A.K., unpublished data). The genome and proteome sequences of S. typhi and S. typhimurium or E. coli were compared in parallel to identify deletions and insertions using the Artemis Comparison Tool (ACT) (K. Rutherford, unpublished data; see also http://www.sanger.ac.uk/Software/ ACT/). Pseudogenes had one or more mutations that would ablate expression, and were identi®ed by direct comparison with S. typhimurium; each of the inactivating mutations was subsequently checked against the original sequencing data.

As part of our effort to sequence the 100-megabase (Mb) genome of the nematode Caenorhabditis elegans, we have completed the nucleotide sequence of a contiguous 2,181,032 base pairs in the central gene cluster of chromosome III. Analysis of the finished sequence has indicated an average density of about one gene per five kilobases; comparison with the public sequence databases reveals similarities to previously known genes for about one gene in three. In addition, the genomic sequence contains several intriguing features, including putative gene duplications and a variety of other repeats with potential evolutionary implications.

Knowledge of the complete genomic DNA sequence of an organism allows a systematic approach to defining its genetic components. The genomic sequence provides access to the complete structures of all genes, including those without known function, their control elements, and, by inference, the proteins they encode, as well as all other biologically important sequences. Furthermore, the sequence is a rich and permanent source of information for the design of further biological studies of the organism and for the study of evolution through cross-species sequence comparison. The power of this approach has been amply demonstrated by the determination of the sequences of a number of microbial and model organisms. The next step is to obtain the complete sequence of the entire human genome. Here we report the sequence of the euchromatic part of human chromosome 22. The sequence obtained consists of 12 contiguous segments spanning 33.4 megabases, contains at least 545 genes and 134 pseudogenes, and provides the first view of the complex chromosomal landscapes that will be found in the rest of the genome.

Salmonella enterica serovars often have a broad host range, and some cause both gastrointestinal and systemic disease. But the serovars Paratyphi A and Typhi are restricted to humans and cause only systemic disease. It has been estimated that Typhi arose in the last few thousand years. The sequence and microarray analysis of the Paratyphi A genome indicates that it is similar to the Typhi genome but suggests that it has a more recent evolutionary origin. Both genomes have independently accumulated many pseudogenes among their approximately 4,400 protein coding sequences: 173 in Paratyphi A and approximately 210 in Typhi. The recent convergence of these two similar genomes on a similar phenotype is subtly reflected in their genotypes: only 30 genes are degraded in both serovars. Nevertheless, these 30 genes include three known to be important in gastroenteritis, which does not occur in these serovars, and four for Salmonella-translocated effectors, which are normally secreted into host cells to subvert host functions. Loss of function also occurs by mutation in different genes in the same pathway (e.g., in chemotaxis and in the production of fimbriae).

Murine gammaherpesvirus 68 (␥HV68) infects mice, thus providing a tractable small-animal model for analysis of the acute and chronic pathogenesis of gammaherpesviruses. To facilitate molecular analysis of ␥HV68 pathogenesis, we have sequenced the ␥HV68 genome. The genome contains 118,237 bp of unique sequence flanked by multiple copies of a 1,213-bp terminal repeat. The GC content of the unique portion of the genome is 46%, while the GC content of the terminal repeat is 78%. The unique portion of the genome is estimated to encode at least 80 genes and is largely colinear with the genomes of Kaposi's sarcoma herpesvirus (KSHV; also known as human herpesvirus 8), herpesvirus saimiri (HVS), and Epstein-Barr virus (EBV). We detected 63 open reading frames (ORFs) homologous to HVS and KSHV ORFs and used the HVS/KSHV numbering system to designate these ORFs. ␥HV68 shares with HVS and KSHV ORFs homologous to a complement regulatory protein (ORF 4), a D-type cyclin (ORF 72), and a G-protein-coupled receptor with close homology to the interleukin-8 receptor (ORF 74). One ORF (K3) was identified in ␥HV68 as homologous to both ORFs K3 and K5 of KSHV and contains a domain found in a bovine herpesvirus 4 major immediate-early protein. We also detected 16 methionine-initiated ORFs predicted to encode proteins at least 100 amino acids in length that are unique to ␥HV68 (ORFs M1 to 14). ORF M1 has striking homology to poxvirus serpins, while ORF M11 encodes a potential homolog of Bcl-2-like molecules encoded by other gammaherpesviruses (gene 16 of HVS and KSHV and the BHRF1 gene of EBV). In addition, clustered at the left end of the unique region are eight sequences with significant homology to bacterial tRNAs. The unique region of the genome contains two internal repeats: a 40-bp repeat located between bp 26778 and 28191 in the genome and a 100-bp repeat located between bp 98981 and 101170. Analysis of the ␥HV68, HVS, EBV, and KSHV genomes demonstrated that each of these viruses have large colinear gene blocks interspersed by regions containing virus-specific ORFs. Interestingly, genes associated with EBV cell tropism, latency, and transformation are all contained within these regions encoding virus-specific genes. This finding suggests that pathogenesis-associated genes of gammaherpesviruses, including ␥HV68, may be contained in similarly positioned genome regions. The availability of the ␥HV68 genomic sequence will facilitate analysis of critical issues in gammaherpesvirus biology via integration of molecular and pathogenetic studies in a small-animal model.