Cell cycle expression of histone genes in Trypanosoma cruzi

In yeast and mammalian cells, the cell cycle-dependent histone genes are ty pically expressed at a 15- to 35-fold higher level during S phase than duri ng other phases of the cell cycle due to increases in both their transcript ion rates (three- to 17-fold) and the stabilities of their mRNAs (three to fivefold). In the protozoan trypanosomatids, most life cycle stage-specific genes are not regulated by changes in transcription rates, but are control led entirely by post-transcriptional events. In contrast, little is known a bout cell cycle-dependent regulation of trypanosomatid genes. To examine ce ll cycle-associated expression of histone genes in a trypanosomatid. Trypan osoma cruzi epimastigotes were synchronized with hydroxyurea. The steady st ate levels of histone mRNAs in the G1, S and G2 phases of the cell cycle we re found to vary only two- to fourfold, peaking in S phase. Nuclear run on assays showed that the histone genes are transcribed by RNA polymerase Il a nd that their transcription rates do not increase in S phase: relative to G 1 and G2. Thus, during S phase of T. cruzi the increase in histone mRNA sta bility is about the same as in mammals and yeast, but no corresponding incr ease in the transcription rates of the histone genes occurs. (C) 2001 Elsev ier Science B.V. All rights reserved.