More surprises from Kinetoplastida

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Abstract
Protozoan parasites of the order Kinetoplastida include various species of the genera Leishmania and Trypanosoma that are responsible for substantial human morbidity and mortality in the tropics. Pathogenic Leishmania species cause a diverse group of diseases, collectively called leishmaniasis, that range in severity from spontaneously healing skin ulcers to fatal visceral disease. African and American trypanosomes cause fatal sleeping sickness and debilitating Chagas disease, respectively. More than a billion people live in areas inhabited by the insects that transmit these parasites, and millions of people are newly infected each year. Organisms of the order Kinetoplastida have a unique organelle called the kinetoplast, an appendix of their single mitochondrion located near the basal body of the flagellum that contains a network of thousands of small interlocking circular DNAs. Kinetoplastids are among the most ancient eukaryotes, with rRNA lineages extending farther back than those of animals, plants, and even fungi (1, 2). As might be expected of such ancient organisms, the kinetoplast is only one of their many distinctive features. The top 10 list of fundamentally important biological phenomena first discovered in Leishmania and trypanosomes includes “programmed” antigenic variation of surface glycoproteins (3), glycosylphosphatidylinositol anchors of membrane proteins (4, 5), expansion/contraction of telomeric DNA repeats (6), bent DNA helices (7), eukaryotic polycistronic transcription (8), trans-splicing of precursor RNAs (9, 10), mitochondrial RNA editing (11), other unique organelles such as glycosomes (12), and distinctive metabolic pathways (13). Several of these phenomena, first unearthed because of their prominence in kinetoplastids, have subsequently been found in higher eukaryotes and have become the focus of intense research interest in those systems. In addition, the many nefarious mechanisms used by Leishmania and trypanosomes to thwart immune defenses thrown at them by their mammalian hosts have led to an enhanced appreciation of the diversity and …