Genome sequence of the endocellular bacterial symbiont of aphids Buchnera sp. APS

Almost all aphid species (Homoptera, Insecta) have 60-80 huge cells called bacteriocytes, within which are round-shaped bacteria that are designated Buchnera. These bacteria are maternally transmitted to eggs and embryos through host generations, and the mutualism between the host and the bacteria is so obligate that neither can reproduce independently. Buchnera is a close relative of Escherichia coli, but it contains more than 100 genomic copies per cell, and its genome size is only a seventh of that of E. coli. Here we report the complete genome sequence of Buchnera sp. strain APS, which is composed of one 640,681-base-pair chromosome and two small plasmids. There are genes for the biosyntheses of amino acids essential for the hosts in the genome, but those for non-essential amino acids are missing, indicating complementarity and syntrophy between the host and the symbiont. In addition, Buchnera lacks genes for the biosynthesis of cell-surface components, including lipopolysaccharides and phospholipids, regulator genes and genes involved in defence of the cell. These results indicate that Buchnera is completely symbiotic and viable only in its limited niche, the bacteriocyte.     

Sequence and analysis of chromosome 5 of the plant Arabidopsis thaliana

The genome of the model plant Arabidopsis thaliana has been sequenced by an international collaboration, The Arabidopsis Genome Initiative. Here we report the complete sequence of chromosome 5. This chromosome is 26 megabases long; it is the second largest Arabidopsis chromosome and represents 21% of the sequenced regions of the genome. The sequence of chromosomes 2 and 4 have been reported previously and that of chromosomes 1 and 3, together with an analysis of the complete genome sequence, are reported in this issue. Analysis of the sequence of chromosome 5 yields further insights into centromere structure and the sequence determinants of heterochromatin condensation. The 5,874 genes encoded on chromosome 5 reveal several new functions in plants, and the patterns of gene organization provide insights into the mechanisms and extent of genome evolution in plants.     

The DNA sequence of human chromosome 21

Chromosome 21 is the smallest human autosome. An extra copy of chromosome 21 causes Down syndrome, the most frequent genetic cause of significant mental retardation, which affects up to 1 in 700 live births. Several anonymous loci for monogenic disorders and predispositions for common complex disorders have also been mapped to this chromosome, and loss of heterozygosity has been observed in regions associated with solid tumours. Here we report the sequence and gene catalogue of the long arm of chromosome 21. We have sequenced 33,546,361 base pairs (bp) of DNA with very high accuracy, the largest contig being 25,491,867 bp. Only three small clone gaps and seven sequencing gaps remain, comprising about 100 kilobases. Thus, we achieved 99.7% coverage of 21q. We also sequenced 281,116 bp from the short arm. The structural features identified include duplications that are probably involved in chromosomal abnormalities and repeat structures in the telomeric and pericentromeric regions. Analysis of the chromosome revealed 127 known genes, 98 predicted genes and 59 pseudogenes.     

The protein-coding sequence of the bovine ACTH-beta-LPH precursor gene is split near the signal peptide region

The pituitary hormones corticotropin (ACTH) and beta-lipotropin (beta-LPH) are formed from a large common precursor. Recently, we have elucidated the whole primary structure of the bovine ACTH-beta-LPH precursor (designated alternatively as preproopiocortin) by determining the nucleotide sequence of cloned DNA complementary to the mRNA coding for the precursor protein. The amino acid sequence assigned has disclosed a characteristic repetitive structure of the ACTH-beta-LPH precursor. The repetitive units of the precursor protein each contain a melanotropin (MSH) sequence (alpha-, beta- or gamma-MSH) as well as other peptide components such as beta-endorphin and corticotropin-like intermediate lobe peptide (CLIP). The repetitive units as well as their peptide components are each bounded by paired basic amino acid residues, which apparently represent the sites of proteolytic processing. Several studies have confirmed the translational initiation site and protein structure assigned (see also ref. 11 and refs therein). In view of the recent knowledge about the organization of eukaryotic genes (see refs 12, 13 for reviews), it would be of particular interest to investigate the relationship between the repetitive structure of the ACTH-beta-LPH precursor containing different functional components and the arrangement of the protein-coding sequence in its gene. We have now isolated and characterized bovine genomic DNA fragments encoding this precursor protein and have demonstrated that the protein sequence is encoded by two non-consecutive DNA segments. An intron (intervening sequence) of approximately 2.2 kilobase pairs separates the smaller exon (mRNA-coding sequence), which contains the gene sequence encoding the signal peptide, from the larger exon, which contains the gene sequence for most of the protein structure, including the known biologically active component peptides.     

Genome sequence of the endocellular obligate symbiont of tsetse flies,Wigglesworthia glossinidia

Many insects that rely on a single food source throughout their developmental cycle harbor beneficial microbes that provide nutrients absent from their restricted diet. Tsetse flies, the vectors of African trypanosomes, feed exclusively on blood and rely on one such intracellular microbe for nutritional provisioning and fecundity. As a result of co-evolution with hosts over millions of years, these mutualists have lost the ability to survive outside the sheltered environment of their host insect cells. We present the complete annotated genome of Wigglesworthia glossinidia brevipalpis, which is composed of one chromosome of 697,724 base pairs (bp) and one small plasmid, called pWig1, of 5,200 bp. Genes involved in the biosynthesis of vitamin metabolites, apparently essential for host nutrition and fecundity, have been retained. Unexpectedly, this obligate's genome bears hallmarks of both parasitic and free-living microbes, and the gene encoding the important regulatory protein DnaA is absent.