Plants synthesize S-methylmethionine (SMM) from S-adenosylmethionine (AdoMet), 
and methionine (Met) by a unique reaction and, like other organisms, use SMM as 
a methyl donor for Met synthesis from homocysteine (Hcy). These reactions 
comprise the SMM cycle. Two Arabidopsis cDNAs specifying enzymes that mediate 
the SMM --> Met reaction (SMM:Hcy S-methyltransferase, HMT) were identified by 
homology and authenticated by complementing an Escherichia coli yagD mutant and 
by detecting HMT activity in complemented cells. Gel blot analyses indicate that 
these enzymes, AtHMT-1 and -2, are encoded by single copy genes. The deduced 
polypeptides are similar in size (36 kDa), share a zinc-binding motif, lack 
obvious targeting sequences, and are 55% identical to each other. The 
recombinant enzymes exist as monomers. AtHMT-1 and -2 both utilize l-SMM or 
(S,S)-AdoMet as a methyl donor in vitro and have higher affinities for SMM. Both 
enzymes also use either methyl donor in vivo because both restore the ability to 
utilize AdoMet or SMM to a yeast HMT mutant. However, AtHMT-1 is strongly 
inhibited by Met, whereas AtHMT-2 is not, a difference that could be crucial to 
the control of flux through the HMT reaction and the SMM cycle. Plant HMT is 
known to transfer the pro-R methyl group of SMM. This enabled us to use 
recombinant AtHMT-1 to establish that the other enzyme of the SMM cycle, 
AdoMet:Met S-methyltransferase, introduces the pro-S methyl group. These 
opposing stereoselectivities suggest a way to measure in vivo flux through the 
SMM cycle.