Genome Structure and Content
Peter J. Myler
from: Leishmania: After The Genome (Edited by: Peter J. Myler and Nicolas Fasel). Caister Academic Press, U.K. (2008)
In the last two years, the genomes of three Leishmania species (L. major, L. infantum and L. braziliensis) have been sequenced, revealing more than 8300 protein-coding and 900 RNA genes. Almost 40% of protein-coding genes fall into 662 families containing between two and 500 members. Most of the smaller gene families are tandem arrays of one to three genes, while the larger gene families are often dispersed in tandem arrays at different loci throughout the genome. Each of the 35 or 36 chromosomes are organized into a small number of gene clusters of tens-to-hundreds of genes on the same DNA strand. These clusters can be organized in head-to-head (divergent) or tail-to-tail (convergent) fashion, with the latter often separated by tRNA, rRNA and/or snRNA genes. Transcription of protein-coding genes initiates bi-directionally in the divergent strand-switch regions between gene clusters and extends polycistronically through each gene cluster before terminating in the strand-switch region separating convergent clusters. Leishmania telomeres are usually relatively small, consisting of a few different types of repeat sequence. Evidence can be found for recombination between several different groups of telomeres. The L. major and L. infantum genomes contain only ~50 copies of inactive degenerated Ingi/L1Tc-related elements (DIREs), while L. braziliensis also contains several telomere-associated transposable elements (TATEs) and spliced leader-associated (SLACs) retroelements. The Leishmania genomes share a conserved core proteome of ~6200 genes with the related trypanosomatids Trypanosoma brucei and Trypanosoma cruzi , but there are ~1000 Leishmania-specific genes (LSGs), which are mostly randomly distributed throughout the genome. There are relatively few (~200) species-specific differences in gene content between the three sequenced Leishmania genomes, but ~8% of the genes appear to be evolving at different rates between the three species, indicative of different selective pressures that could be related to disease pathology. While these genome sequences have vastly increased our knowledge of Leishmania genome content and organization, much work remains to be done, since ~65% of protein-coding genes currently lack functional assignment read more ...