<jats:title>Summary</jats:title><jats:p>HMm is the only archaeal histone in <jats:italic>Methanosarcina mazei</jats:italic> Göl and recombinant HMm, synthesized by expression of MM1825 in <jats:italic>Escherichia coli</jats:italic>, has been purified and confirmed to have the DNA binding and compaction properties characteristic of an archaeal histone. Insertion of a puromycin resistance conferring cassette (<jats:italic>pac</jats:italic>) into MM1825 was not lethal but resulted in mutants (<jats:italic>M. mazei</jats:italic> MM1825<jats:italic>::pac</jats:italic>) that have impaired ability to grow on methanol and trimethylamine. Loss of HMm also resulted in increased sensitivity to UV light and decreased transcript levels for ∼25% of all <jats:italic>M. mazei</jats:italic> genes. For most genes, the transcript decrease was 3‐ to 10‐fold, but transcripts of MM483 (small heat‐shock protein), MM1688 (trimethylamine:corrinoid methyl transferase) and MM3195 (transcription regulator), were reduced 100‐, 100‐ and 25‐fold, respectively, in <jats:italic>M. mazei</jats:italic> MM1825<jats:italic>::pac</jats:italic> cells. Transcripts of only five adjacent genes that appear to constitute an aromatic amino acid biosynthetic operon were elevated in <jats:italic>M. mazei</jats:italic> MM1825<jats:italic>::pac</jats:italic> cells. Complementary synthesis of HMm from a plasmid transformed into <jats:italic>M. mazei</jats:italic> MM1825::<jats:italic>pac</jats:italic> restored wild‐type growth and transcript levels.</jats:p>