Conservation of an Ancient Oxidation Response That Controls Archaeal Epigenetic Traits Through Chromatin Protein Networks

Epigenetic variants of the archaeon Sulfolobus solfataricus called SARC have evolved heritable traits including extreme acid resistance, enhanced genome integrity and a conserved \"SARC\" transcriptome related to acid resistance. These traits appear to result from altered chromatin protein function related to the heritable hypomethylation of chromatin proteins Cren7 and Sso7D. To clarify how this might occur, ChIPseq and Affinity Purification Mass Spectrometry (AP-MS) were used to compare Cren7 and Sso7D genome binding sites and protein networks between lineages (wild type and SARC) and culture pH (pH 1 and 3). All SARC transcriptome loci were bound by these chromatin proteins but with invariant patterns indicating binding alone was insufficient to mediate the SARC traits. In contrast, chromosome association varied at other loci. Quantitative AP-MS was then used to identify protein interaction networks and these included transcription and DNA repair proteins implicated in the evolved heritable traits that varied in abundance between SARC and wild type strains. Protein networks included most of the S-adenosylmethionine (SAM) synthesis pathway including serine hydroxymethyltransferase (SHMT), whose abundance varied widely with culture pH. Because epigenetic marks are coupled to SAM pools and oxidative stress in eukaryotes, occurrence of a similar process was investigated here. Archaeal SAM pools were depleted by treatment with SAM pathway inhibitors, acid or oxidative stress and, like eukaryotes, levels were raised by vitamin B12 and methionine supplementation. We propose that in archaea, oxidation-induced SAM pool depletion acting through an SHMT sensor, drove chromatin protein hypomethylation and thereby protein network changes that established the evolved SARC epigenetic traits.\n\nSignificance StatementArchaea and eukaryotes share many molecular processes, including chromatin-mediated epigenetic inheritance of traits. As with eukaryotes, archaeal protein complexes were formed between trait-related proteins and chromatin proteins, subject to chromatin protein methylation state. Oxidation-induced depletion of S-adenosylmethionine (SAM) pools likely resulted in chromatin protein hypomethylation. Subsequent chromatin enrichment of serine hydroxymethyltransferase as a response to oxidative stress could modulate methylation at specific genomic loci. The interplay between archaeal metabolism and chromatin appear consistent with patterns observed in eukaryotes and indicate the existence of an ancient oxidation signal transduction pathway controlling epigenetics.