S-Adenosylmethionine decarboxylase from the archaeon Methanococcus jannaschii: Identification of a novel family of pyruvoyl enzymes

Polyamines are present in high concentrations in archaea, get little is kno wn about their synthesis, except by extrapolation from bacterial and eucary al systems. S-Adenosylmethionine (AdoMet) decarboxylase, a pyruvoyl group-c ontaining enzyme that is required for spermidine biosynthesis, has been pre viously identified in eucarya and Escherichia coli. Despite spermidine conc entrations in the Methanococcales that are several times higher than in E. coli, no AdoMet decarboxylase gene was recognized in the complete genome se quence of Methanococcus jannaschii. The gene encoding AdoMet decarboxylase in this archaeon is identified herein as a highly diverged homolog of the E . coli speD gene (less than 11% identity). The M. jannaschii enzyme has bee n expressed in E. coli and purified to homogeneity, Mass spectrometry showe d that the enzyme is composed of two subunits of 61 and 63 residues that ar e derived from a common proenzyme; these proteins associate in an (alpha be ta)(2) complex. The pyruvoyl-containing subunit is less than one-half the s ize of that in previously reported AdoMet decarboxylases, but the holoenzym e has enzymatic activity comparable to that of other AdoMet decarboxylases. The sequence of the M,jannaschii enzyme is a prototype of a class of AdoMe t decarboxylases that includes homologs in other archaea and diverse bacter ia. The broad phylogenetic distribution of this group suggests that the can onical SpeD-type decarboxylase was derived from an archaeal enzyme within t he gamma proteobacterial lineage. Both SpeD-type and archaeal-type enzymes have diverged widely in sequence and size from analogous eucaryal enzymes.