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.