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Identification of a phenolic 3-O-methyltransferase in the lignin-degrading fungus Phanerochaete chrysosporium

¹ Microbial Biochemistry Section, Department of Food Science, The Queen's University of Belfast, Newforge Lane, Belfast BT9 5PX, UK
² Food Science Division, Department of Agriculture for Northern Ireland, Newforge Lane, Belfast BT9 5PX, UK

Author for correspondence: David B. Harper. Tel: +44 1232 255343. Fax: + 44 1232 669551. e-mail: Harper_David/Science_FSD_Newforge@dani.gov.uk

ABSTRACT

A methyl transferase enzyme catalysing the 3-O-methylation of isovanillic acid (3-hydroxy-4-methoxy!benzoic acid) by S-adenosylmethionine (SAM) was identified in Phanerochaete chrysosporium and purified. Gel filtration indicated an M_r of 71000 and SDS-PAGE showed that the enzyme was composed of two sublimits of M_r approximately 36000. Substrate utilization studies demonstrated that the enzyme was highly specific, displaying an exclusive preference for the methylation of the 3-hydroxyl group of several substituted benzoic acids. 3-Hydroxybenzoic acids with a methoxyl or hydroxyl substituent in the 2 or 4 position were the best substrates with isovanillic and 3,4-dihydroxybenzoic acids showing the highest rates of methylation. The 3-O-methyltransferase enzyme was induced later in the growth cycle than the 4-O-methyltransferase previously isolated from this fungus, which is believed to have a role in the 4-O-methylation of lignin degradation products. However the function of this meta-specific enzyme, the first phenolic 3-O-methyltransferase isolated from a fungus, remains unclear. The combined activities of the 3- and 4-O-methyltransferase enzymes satisfactorily account for the pattern of SAM-dependent methylating activity shown by whole mycelia to phenolic substrates.

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