HMm is the only archaeal histone in Methanosarcina mazei Göl and recombinant HMm, synthesized by expression of MM1825 in Escherichia coli, 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 (pac) into MM1825 was not lethal but resulted in mutants (M. mazei MM1825::pac) 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 M. mazei 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 M. mazei MM1825::pac cells. Transcripts of only five adjacent genes that appear to constitute an aromatic amino acid biosynthetic operon were elevated in M. mazei MM1825::pac cells. Complementary synthesis of HMm from a plasmid transformed into M. mazei MM1825::pac restored wild-type growth and transcript levels.