Chromatin-based transcriptional punctuation

The long polycistronic transcription units of trypanosomes do not appear to be demarcated by the usual DNA motifs that punctuate transcription in familiar eukaryotes. In this issue of Genes & Development, Siegel and colleagues (pp. 1063-1076) describe a system for the demarcation of trypanosome transcription units based on the deposition and turnover of histone variants rather than on the binding of transcription factors. Replication-independent incorporation of histone variants and destabilization of nucleosomes is an emerging theme at promoters of more familiar eukaryotes, and it now appears that this system is an evolutionarily conserved mode of transcriptional punctuation.