ORGANIZATION, TRANSCRIPTION AND REGULATION OF THE LEISHMANIA-INFANTUMHISTONE H3 GENES

The genomic organization and transcription of the genes encoding the h istone H3 of the protozoan parasite Leishmania infantum have been stud ied. It was found that there are multiple copies of the histone H3 gen es distributed in chromosomal bands XIX and XIV. The nucleotide sequen ce of two of the L. infantum H3 genes, each one located in a different chromosome, is reported. Although the nucleotide sequence of the codi ng region of both genes is identical, the sequence of the 3' untransla ted region is highly divergent. It was found also that there exist two different size classes of histone H3 transcripts, each one derived fr om a different gene, and that they are polyadenylated. The steady-stat e level of the transcripts dramatically decreases when the parasites e nter the stationary phase of growth, suggesting a mode of regulation w hich is linked to the proliferation status of the cell. Unlike the rep lication-dependent histones, the L. infantum H3 mRNA levels do not dec rease after treatment with DNA synthesis inhibitors. A comparative ana lysis of the sensitivity of the histone mRNA levels to DNA inhibition in the parasites L. infantum and Trypanosoma cruzi revealed the existe nce of different control mechanisms in histone expression in these two phylogenetically related protozoan parasites.