Structure of Alba: an Archaeal Chromatin Protein Modulated by Acetylation

Eukaryotic DNA is packaged into nucleosomes that regulate the accessibility of the genome to replication, transcription and repair factors. Chromatin accessibility is controlled by histone modifications including acetylation and methylation. Archaea possess eukary otic-like machineries for DNA replication, transcription and information processing. The conserved archaeal DNA binding protein Alba (formerly Sso10b) interacts with the silencing protein Sir2, which regulates Alba's DNA binding affinity by deacetylation of a lysine residue. We present the crystal structure of Alba from Sulfolobus solfataricus at 2.6 A resolution (PDB code 1h0x). The fold is reminiscent of the N-terminal DNA binding domain of DNase I and the C-terminal domain of initiation factor IF3. The Alba dimer has two extended beta-hairpins flanking a central body containing the acetylated lysine, Lys16, suggesting three main points of contact with the DNA. Fluorescence, calorimetry and electrophoresis data suggest a final binding stoichiometry of approximately 5 bp DNA per Alba dimer. We present a model for the Alba-DNA interaction consistent with the available structural, biophysical and electron microscopy data.