Formaldehyde activating enzyme (Fae) was first discovered in methylotrophic bacteria, where it is involved in the oxidation of methanol to CO2 and in formaldehyde detoxification. The 18 kDa protein catalyzes the condensation of formaldehyde with tetrahydromethanopterin (H4MPT) to methylene-H4MPT. We describe here that Fae is also present and functional in the methanogenic archaeon Methanosarcina barkeri. The faeA homologue in the genome of M. barkeri was heterologously expressed in Escherichia coli and the overproduced purified protein shown to actively catalyze the condensation reaction: apparent Vmax = 13 U/mg protein (1 U = micromol/min); apparent Km for H4MPT = 30 microM; apparent Km for formaldehyde = 0.1 mM. By Western blot analysis the concentration of Fae in cell extracts of M. barkeri was determined to be in the order of 0.1% of the soluble cell proteins. Besides the faeA gene the genome of M. barkeri harbors a second gene, faeB-hpsB, which is shown to code for a 42 kDa protein with both Fae activity (3.6 U/mg) and hexulose-6-phosphate synthase (Hps) activity (4.4 U/mg). The results support the recent proposal that in methanogenic archaea Fae and Hps could have a function in ribose phosphate synthesis.