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Background  Histone synthesis in  Leishmania  is tightly coupled to DNA replication by a post-transcriptional mechanism operating at the level of translation.    Results  In this work we have analyzed the implication of the 5' and 3' untranslated regions (UTR) in the cell cycle regulated expression of the histone  H2A  in  Leishmania infantum . For that purpose,  L. infantum  promastigotes were stably transfected with different plasmid constructs in which the  CAT  coding region used as a reporter was flanked by the 5' and 3' UTR regions of the different  H2A  genes. We report that in spite of their sequence differences, histone  H2A  5' and 3' UTRs conferred a cell cycle dependent pattern of expression on the CAT reporter since  de novo  synthesis of CAT increased when parasites enter the S phase. Using one established  L. infantum  cell line we showed that CAT expression is controlled by the same regulatory events that control the endogenous histone gene expression. Thus, although we did not detect changes in the level of  CAT  mRNAs during cell cycle progression, a drastic change in the polysome profiles of  CAT  mRNAs was observed during the progression from G1 to S phase. In the S phase  CAT  mRNAs were on polyribosomal fractions, but in the G1 phase the association of  CAT  transcripts with ribosomes was impaired. Furthermore, it was determined that the addition of just the  H2A  3' UTR to the  CAT  reporter gene is sufficient to achieve a similar pattern of post-transcriptional regulation indicating that this region contains the major regulatory sequences involved in the cell cycle dependent expression of the  H2A  genes. On the other hand, although CAT transcripts bearing the  H2A  5' alone were translated both in the G1 and S phase, higher percentages of transcripts were detected on polyribosomes in the S phase correlating with an increase in the  de novo  synthesis of CAT. Thus, it can be concluded that this region also contributes, although to a minor extent than the 3' UTR, in the enhancement of translation in the S phase relative to the G1 phase.    Conclusion  Our findings indicate that both, the 5' and the 3' UTRs contain sequence elements that contribute to the cell cycle expression of  L. infantum  H2A. The 3' UTR region is essential for cell cycle dependent translation of the  L. infantum  H2A transcripts whereas the 5' UTR has a minor contribution in their S phase dependent translation.

bmc molecular biology

Key role of the 3' untranslated region in the cell cycle regulated expression of the Leishmania infantum histone H2A genes: minor synergistic effect of the 5' untranslated region