Histone acetylations mark origins of polycistronic transcription in Leishmania major

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Abstract
Background: Many components of the RNA polymerase II transcription machinery have been identified in kinetoplastid protozoa, but they diverge substantially from other eukaryotes. Furthermore, protein-coding genes in these organisms lack individual transcriptional regulation, since they are transcribed as long polycistronic units. The transcription initiation sites are assumed to lie within the 'divergent strand-switch' regions at the junction between opposing polycistronic gene clusters. However, the mechanism by which Kinetoplastidae initiate transcription is unclear, and promoter sequences are undefined. Results: The chromosomal location of TATA-binding protein (TBP or TRF4), Small Nuclear Activating Protein complex (SNAP50), and H3 histones were assessed in Leishmania major using microarrays hybridized with DNA obtained through chromatin immunoprecipitation (ChIP-chip). The TBP and SNAP50 binding patterns were almost identical and high intensity peaks were associated with tRNAs and snRNAs. Only 184 peaks of acetylated H3 histone were found in the entire genome, with substantially higher intensity in rapidly-dividing cells than stationary-phase. The majority of the acetylated H3 peaks were found at divergent strand-switch regions, but some occurred at chromosome ends and within polycistronic gene clusters. Almost all these peaks were associated with lower intensity peaks of TBP/SNAP50 binding a few kilobases upstream, evidence that they represent transcription initiation sites. Conclusion: The first genome-wide maps of DNA-binding protein occupancy in a kinetoplastid organism suggest that H3 histones at the origins of polycistronic transcription of protein-coding genes are acetylated. Global regulation of transcription initiation may be achieved by modifying the acetylation state of these origins.

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