β-alanine N-methyltransferase of Limonium latifolium. cDNA cloning and functional expression of a novel N-methyltransferase implicated in the synthesis of the osmoprotectant β-alanine betaine

Abstract

μ-alanine (Ala) betaine, an osmoprotectant suitable under saline and hypoxic environments, is found in most members of the halophytic plant family Plumbaginaceae. In Limonium latifolium (Plumbaginaceae), it is synthesized via methylation of β-Ala by the action of a trifunctional S-adenosyl L-methionine (Ado-Met): β-Ala N-methyltransferase (NMTase). Peptide sequences from purified β-Ala NMTase were used to design primers for reverse transcriptase-PCR, and several cDNA clones were isolated. The 5′ end of the cDNA was cloned using a 5′-rapid amplification of cDNA ends protocol. A 500-bp cDNA was used as a probe to screen a λ-gt10 L. latifolium leaf cDNA library. Partial cDNA clones represented two groups, NMTase A and NMTase B, differing only in their 3′-untranslated regions. The full-length NMTase A cDNA was 1,414 bp and included a 1128-bp open reading frame and a 119-bp 5′-untranslated region. The deduced amino acid sequence of 375 residues had motifs known to be involved in the binding of Ado-Met. The NMTase mRNA was expressed in L. latifolium leaves but was absent in Limonium sinuatum, a member of the genus that lacks the synthetic pathway for β-Ala betaine. NMTase mRNA expression was high in young and mature leaves and was enhanced by light. NMTase cDNA was expressed in yeast (Saccharomyces cerevisiae) under the control of a galactose-inducible promoter. Protein extracts of galactose-induced recombinant yeast had Ado-Met-specific NMTase activities that were highly specific to β-Ala, N-methyl β-Ala, and N,N-dimethyl β-Ala as methyl acceptors. NMTase activities were not detectable in comparable protein extracts of yeast, transformed with vector control. The NMTase protein sequence shared homology with plant caffeic acid O-methyltransferases and related enzymes. Phylogenetic analyses suggested that β-Ala NMTase represents a novel family of N-methyltransferases that are evolutionarily related to O-methyltransferases.