title: Insertional inactivation of the methionine s-methyltransferase gene eliminates the s-methylmethionine cycle and increases the methylation ratio.
abstract: Methionine (Met) S-methyltransferase (MMT) catalyzes the synthesis of S-methyl-Met (SMM) from Met and S-adenosyl-Met (Ado-Met). SMM can be reconverted to Met by donating a methyl group to homocysteine (homo-Cys), and concurrent operation of this reaction and that mediated by MMT sets up the SMM cycle. SMM has been hypothesized to be essential as a methyl donor or as a transport form of sulfur, and the SMM cycle has been hypothesized to guard against depletion of the free Met pool by excess Ado-Met synthesis or to regulate Ado-Met level and hence the Ado-Met to S-adenosylhomo-Cys ratio (the methylation ratio). To test these hypotheses, we isolated insertional mmt mutants of Arabidopsis and maize (Zea mays). Both mutants lacked the capacity to produce SMM and thus had no SMM cycle. They nevertheless grew and reproduced normally, and the seeds of the Arabidopsis mutant had normal sulfur contents. These findings rule out an indispensable role for SMM as a methyl donor or in sulfur transport. The Arabidopsis mutant had significantly higher Ado-Met and lower S-adenosylhomo-Cys levels than the wild type and consequently had a higher methylation ratio (13.8 versus 9.5). Free Met and thiol pools were unaltered in this mutant, although there were moderate decreases (of 30%-60%) in free serine, threonine, proline, and other amino acids. These data indicate that the SMM cycle contributes to regulation of Ado-Met levels rather than preventing depletion of free Met.
authors.affiliation: Horticultural Sciences Department, University of Florida, Gainesville 32611, USA.
authors.forename: Michael G
authors.initials: MG
authors.lastname: Kocsis
authors.forename: Philippe
authors.initials: P
authors.lastname: Ranocha
authors.forename: Douglas A
authors.initials: DA
authors.lastname: Gage
authors.forename: Eric S
authors.initials: ES
authors.lastname: Simon
authors.forename: David
authors.initials: D
authors.lastname: Rhodes
authors.forename: Gregory J
authors.initials: GJ
authors.lastname: Peel
authors.forename: Stefan
authors.initials: S
authors.lastname: Mellema
authors.forename: Kazuki
authors.initials: K
authors.lastname: Saito
authors.forename: Motoko
authors.initials: M
authors.lastname: Awazuhara
authors.forename: Changjiang
authors.initials: C
authors.lastname: Li
authors.forename: Robert B
authors.initials: RB
authors.lastname: Meeley
authors.forename: Mitchell C
authors.initials: MC
authors.lastname: Tarczynski
authors.forename: Conrad
authors.initials: C
authors.lastname: Wagner
authors.forename: Andrew D
authors.initials: AD
authors.lastname: Hanson
mesh_terms: D000596:Amino Acids / Q000032:analysis; D017360:Arabidopsis / Q000737:chemistry / Q000201:enzymology / Q000235:genetics / Q000378:metabolism*; D008745:Methylation; D008780:Methyltransferases / Q000235:genetics* / Q000378:metabolism*; D016254:Mutagenesis, Insertional; D012435:S-Adenosylhomocysteine / Q000032:analysis; D012436:S-Adenosylmethionine / Q000032:analysis; D012639:Seeds / Q000737:chemistry / Q000254:growth & development / Q000378:metabolism; D013438:Sulfhydryl Compounds / Q000032:analysis; D013455:Sulfur / Q000032:analysis; D014814:Vitamin U / Q000378:metabolism*; D003313:Zea mays / Q000737:chemistry / Q000201:enzymology / Q000235:genetics / Q000378:metabolism*
chemical_list: D000596:Amino Acids; D013438:Sulfhydryl Compounds; D014814:Vitamin U; D013455:Sulfur; D012436:S-Adenosylmethionine; D012435:S-Adenosylhomocysteine; D008780:Methyltransferases; C093871:methionine S-methyltransferase