Leishmania major Friedlin chromosome 1 has an unusual distribution of protein-coding genes

Leishmania are evolutionarily ancient protozoans (Kinetoplastidae) and impo rtant human pathogens that cause a spectrum of diseases ranging from the as ymptomatic to the lethal. The Leishmania genome is relatively small [approx imate to 34 megabases (Mb)], lacks substantial repetitive DNA, and is distr ibuted among 36 chromosomes pairs ranging in size from 0.3 Mb to 2.5 Mb, ma king it a useful candidate for complete genome sequence determination. We r eport here the nucleotide sequence of the smallest chromosome, chr1. The se quence of chr1 has a 257-kilobase region that is densely packed with 79 pro tein-coding genes. This region is flanked by telomeric and subtelomeric rep etitive elements that vary in number and content among the chr1 homologs, r esulting in an approximate to 27.5-kilobase size difference. Strikingly, th e first 29 genes are all encoded on one DNA strand, whereas the remaining 5 0 genes are encoded on the opposite strand. Based on the gene density of ch r1, ne predict a total of approximate to 9,800 genes in Leishmania, of whic h 40% may encode unknown proteins.