Structure and regulation of histone H2B mRNAs from Leishmania enriettii.

Abstract

We have studied the structure and expression of histone H2B mRNAs and genes in the parasitic protozoan Leishmania enriettii. A genomic clone containing three tandemly repeated genes has been sequenced and shown to encode three identical histone proteins and two types of closely related mRNA sequence. We have also sequenced three independent cDNA clones and demonstrated that the Leishmania H2B mRNAs are polyadenylated, similar to the basal histone mRNAs of higher eucaryotes and the histone mRNAs of yeast. In addition, the Leishmania mRNAs contain inverted repeats near the poly(A) tail which could form stem-loops similar in secondary structure, but not in sequence, to the 3' stem-loops of nonpolyadenylated replication-dependent histones of higher eucaryotes. Unlike the replication-dependent histones, the Leishmania histone H2B mRNAs do not decrease in abundance following treatment with inhibitors of DNA synthesis. The histone mRNAs are differentially expressed during the parasite life cycle and accumulate to a higher level in the extracellular promastigotes (the form which in nature lives within the gut of the insect vector) than in the intracellular amastigotes (the form that lives within the mammalian host macrophages).Leishmania spp. are parasitic protozoa with two principle stages in their life cycle (52). The promastigote is a flagellated, extracellular organism that colonizes the alimentary tract of the sand fly vector. Promastigotes, which are injected from the insect salivary glands into a vertebrate host by a bite by the infected insect, are phagocytized by the host macrophages; within the lysosomes of the macrophages, the parasites transform into the other life cycle stage, the amastigotes. The oval, nonflagellated amastigotes are specialized for survival within the macrophage lysosomes, where they divide, lyse the host cell, and reinvade new macrophages. The life cycle is completed when a sand fly ingests the infected macrophages during a blood meal and the parasites retransform into the promastigote forms within the insect gut.Leishmania spp. and other related kinetoplastid protozoa are among the most primitive eucaryotes; phylogenetic studies have placed them well below either fission or budding yeasts on an evolutionary scale (45). As a reflection of this phylogenetic divergence, these organisms possess many unusual features of gene expression. trans splicing of mRNAs (1,33,49), transcriptional editing of mitochondrial RNAs (42), and polycistronic mRNA precursors (32) are features found commonly among the kinetoplastid protozoa but not observed as frequently in other eucaryotes. Other novel mechanisms for expressing or regulating the expression of genetic information are likely to be discovered in these protozoa.Histones are proteins that are ubiquitous among eucaryotes (24). These small basic polypeptides constitute five classes of protein which are essential elements of eucaryotic chromatin: histones Hi, H2A, H2B, H3, and H4. Two copies each of histones H2A, H2B, H3, and H4 form an octamer around w...

Coupling of poly(A) site selection and trans-splicing in Leishmania.

“…Because poly(A) addition within the NEO-coding region is selected against in these experiments, the smaller spacings may not necessarily reflect the optimal location of a poly(A) site but, rather, may reflect constraints placed on the system by the proximity of the NEO gene. A survey of intergenic regions in Leishmania whose poly(A) and splice sites have been mapped reveals spacings from 85 to 660 nucleotides, with an average of 383 and a median of 390 (nine loci; Landfear et al 1986;Lee et al 1988;Kapler et al 1990a;Genske et al 1991;Flinn and Smith 1992). These values are consistent with the spacing inferred from Northern and Southern blot analyses of the HSP83, gp63, and A/D1/D2/B/C loci (Button et al 1989;Shapira and Pinelli 1989;Flinn and Smith 1992;Ramamoorthy et al 1992).…”

The Translational Efficiencies of the Two Leishmania infantum HSP70 mRNAs, Differing in Their 3′-Untranslated Regions, Are Affected by Shifts in the Temperature of Growth through Different Mechanisms

“…Another peculiarity of gene organization in Leishmania is that relevant housekeeping genes are repeated, forming tandem arrays that shares total (or almost total) sequence conservation in the coding regions but showing untranslated regions with high sequence divergence (4,30). Examples are genes coding for ribosomal proteins (31,32), histones (29,33), and phosphoglycerate kinase (34). Also, genes showing this peculiar type of genomic organization have been found on several L. major chromosomes (35,36).…”

Identification and Characterization of T Cell-Stimulating Antigens from Leishmania by CD4 T Cell Expression Cloning

“…Cultures were pulsed with [ 3 H]thymidine for the last 18 h. Cultures were performed in triplicate and the figure is representative of three separate experiments. these Ags have been previously shown to induce T cell responses in leishmaniasis patients, although histone H2b, ␣-and ␤-tubulin, as well as malate dehydrogenase have been previously characterized in terms of biochemistry or molecular biology (27)(28)(29)(30).…”