Transcriptional activation in the context of repression mediated by archaeal histones.

Many archaea (including all the methanogens, nearly all euryarchaeotes, and some crenarchaeotes) use histones as components of the chromatin that compacts their genomes. The archaeal histones are homo- and heterodimers that pair on DNA to form tetrasomes (as the eukaryotic histones H3 and H4 do). The resulting DNA packaging is known to interfere with assembly of the archaeal transcription apparatus at promoters; the ability of transcriptional activation to function in repressive archaeal chromatin has not yet been explored in vitro. Using four of the Methanocaldococcus jannaschii (Mja) histones, we have examined activation of the model Mja rb2 transcription unit by the Mja transcriptional activator Ptr2 in this simplified-chromatin context. Using hydroxyl radical footprinting, we find that the Ptr2-specific rb2 upstream activating site is a preferred histone-localizing site that nucleates histone: DNA-binding radiating from the rb2 promoter. Nevertheless, Ptr2 competes effectively with histones for access to the rb2 promoter and most potently activates transcription in vitro at histone concentrations that extensively coat DNA and essentially silence basal transcription.