Characterization of selenocysteine methyltransferases from Astragalus species with contrasting selenium accumulation capacity

Thomas G. Sors, Catherine P. Martin, David E. Salt

Research output: Contribution to journalArticle

36 Citations (Scopus)

Abstract

P>A group of selenium (Se)-hyperaccumulating species belonging to the genus Astragalus are known for their capacity to accumulate up to 0.6% of their foliar dry weight as Se, with most of this Se being in the form of Se-methylselenocysteine (MeSeCys). Here, we report the isolation and molecular characterization of the gene that encodes a putative selenocysteine methyltransferase (SMT) enzyme from the non-accumulator Astragalus drummondii and biochemically compare it with an authentic SMT enzyme from the Se-hyperaccumulator Astragalus bisulcatus, a related species that lives within the same native habitat. The non-accumulator enzyme (AdSMT) shows a high degree of homology with the accumulator enzyme (AbSMT) but lacks the selenocysteine methyltransferase activity in vitro, explaining why little or no detectable levels of MeSeCys accumulation are observed in the non-accumulator plant. The insertion of mutations on the coding region of the non-accumulator AdSMT enzyme to better resemble enzymes that originate from Se accumulator species results in increased selenocysteine methyltransferase activity, but these mutations were not sufficient to fully gain the activity observed in the AbSMT accumulator enzyme. We demonstrate that SMT is localized predominantly within the chloroplast in Astragalus, the principal site of Se assimilation in plants. By using a site-directed mutagenesis approach, we show that an Ala to Thr amino acid mutation at the predicted active site of AbSMT results in a new enzymatic capacity to methylate homocysteine. The mutated AbSMT enzyme exhibited a sixfold higher capacity to methylate selenocysteine, thereby establishing the evolutionary relationship of SMT and homocysteine methyltransferase enzymes in plants.

Original languageEnglish
Pages (from-to)110-122
Number of pages13
JournalThe Plant Journal
Volume59
Issue number1
DOIs
Publication statusPublished - Jul 2009

Keywords

  • Astragalus bisulcatus
  • Astragalus drummondii
  • selenium accumulation
  • selenocysteine methyltransferase
  • homocysteine methyltransferase
  • Se-methylselenocysteine
  • sulfur
  • methionine synthase
  • plants
  • homocysteine
  • tolerance
  • arabidopsis
  • assimilation
  • bisulcatus
  • expression

Cite this

Characterization of selenocysteine methyltransferases from Astragalus species with contrasting selenium accumulation capacity. / Sors, Thomas G.; Martin, Catherine P.; Salt, David E.

In: The Plant Journal, Vol. 59, No. 1, 07.2009, p. 110-122.

Research output: Contribution to journalArticle

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AB - P>A group of selenium (Se)-hyperaccumulating species belonging to the genus Astragalus are known for their capacity to accumulate up to 0.6% of their foliar dry weight as Se, with most of this Se being in the form of Se-methylselenocysteine (MeSeCys). Here, we report the isolation and molecular characterization of the gene that encodes a putative selenocysteine methyltransferase (SMT) enzyme from the non-accumulator Astragalus drummondii and biochemically compare it with an authentic SMT enzyme from the Se-hyperaccumulator Astragalus bisulcatus, a related species that lives within the same native habitat. The non-accumulator enzyme (AdSMT) shows a high degree of homology with the accumulator enzyme (AbSMT) but lacks the selenocysteine methyltransferase activity in vitro, explaining why little or no detectable levels of MeSeCys accumulation are observed in the non-accumulator plant. The insertion of mutations on the coding region of the non-accumulator AdSMT enzyme to better resemble enzymes that originate from Se accumulator species results in increased selenocysteine methyltransferase activity, but these mutations were not sufficient to fully gain the activity observed in the AbSMT accumulator enzyme. We demonstrate that SMT is localized predominantly within the chloroplast in Astragalus, the principal site of Se assimilation in plants. By using a site-directed mutagenesis approach, we show that an Ala to Thr amino acid mutation at the predicted active site of AbSMT results in a new enzymatic capacity to methylate homocysteine. The mutated AbSMT enzyme exhibited a sixfold higher capacity to methylate selenocysteine, thereby establishing the evolutionary relationship of SMT and homocysteine methyltransferase enzymes in plants.

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KW - sulfur

KW - methionine synthase

KW - plants

KW - homocysteine

KW - tolerance

KW - arabidopsis

KW - assimilation

KW - bisulcatus

KW - expression

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VL - 59

SP - 110

EP - 122

JO - The Plant Journal

JF - The Plant Journal

SN - 0960-7412

IS - 1

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