Insights into the post-translational modifications of archaeal Sis10b (Alba): lysine-16 is methylated, not acetylated, and this does not regulate transcription or growth.

Nucleic acid-binding proteins of the Sac10b family, also referred to as Alba (for acetylation lowers binding affinity), are highly conserved in Archaea. It was reported that Sso10b, a Sac10b homologue from Sulfolobus solfataricus, was acetylated at the ɛ-amino group of K16 and the α-amino group of the N-terminal residue. Notably, acetylation of K16 reduced the affinity of Sso10b for DNA and de-repressed transcription in vitro. Here, we show that Sis10b, a Sac10b homologue from Sulfolobus islandicus, underwent a range of post-translational modifications (PTMs). K16 in Sis10b as well as Sso10b was not acetylated. Substitution of K16 for R16, which resulted in the loss of the PTMs at the site, showed little effect on the growth of the cell and resulted in only a slight change in the expression of a very small fraction of the genes. The N-terminus of Sis10b was nearly completely Nα -acetylated. The reduction or loss of the terminal acetylation led to a significant increase in the cellular concentration of Sis10b, suggesting the involvement of the modification in the control of the turnover of the protein. These results have clarified the PTMs of Sac10b homologues and shed light on the proposed roles of acetylation of the protein.