Comparative Genomics of Leishmania Parasites (pp. 59-80)
Authors: (Norma E. Padilla-Mejía, Claudia M. Gómez-Hurtado, Inti I. Sánchez-Santamaría, Luis E. Florencio-Martínez, Rebeca G. Manning-Cela and Santiago Martínez-Calvillo, Unidad de Biomedicina, Facultad de Estudios Superiores Iztacala, Universidad Nacional Autónoma de México, Mexico, and others)
Abstract: The parasites of the genus Leishmania are trypanosomatid protozoa that produce a
spectrum of diseases ranging from mild skin or mucosal lesions to fatal visceral
leishmaniasis. The genome sequences of five different species of Leishmania have been
reported to date: L. major, L. infantum, L. braziliensis, L. mexicana and L. donovani.
Analyses of the sequences revealed that the genomes of these parasites are organized into
large directional gene clusters, i.e. tens-to-hundreds of protein-coding genes arranged
sequentially on the same strand of DNA. A remarkable conservation of gene order
(synteny) was observed in the genomes of the different Leishmania species. Interestingly,
an unexpectedly small number of species-specific genes was identified. However, the
analyses showed gene and chromosome copy number differences between species,
indicating that increased gene copy number may cause changes in gene expression that
might influence disease tropism. Contrary to what occurs in other eukaryotes,
transcription in Leishmania and other trypanosomatids is polycistronic, and mature
messenger RNAs (mRNAs) are generated from primary transcripts by trans-splicing and
polyadenylation. Little is known about either the DNA sequences or the proteins that are
involved in transcription initiation in Leishmania. Bioinformatic analyses of the genome
databases of these parasites led to the identification of a small number of proteins
involved in gene expression. However, functional studies have revealed that
trypanosomatids have more general transcription factors than originally estimated.
Analysis of the Leishmania databases showed the presence of a relatively low number of
transfer RNA (tRNA) genes, which are organized into clusters of two to 10 genes that
may contain other Pol III-transcribed genes. Also, with the exception of L. braziliensis,
the Leishmania genome does not contain active retrotransposons. Interestingly, L.
braziliensis possesses components of an RNA-mediated interference pathway, which is
not present in other Leishmania species.