A methyl transferase enzyme catalysing the 3--methylation of isovanillic acid (3-hydroxy-4-methoxy!benzoic acid) by -adenosylmethionine (SAM) was identified in and purified. Gel filtration indicated an of 71000 and SDS-PAGE showed that the enzyme was composed of two sublimits of M 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--methyltransferase enzyme was induced later in the growth cycle than the 4--methyltransferase previously isolated from this fungus, which is believed to have a role in the 4--methylation of lignin degradation products. However the function of this -specific enzyme, the first phenolic 3--methyltransferase isolated from a fungus, remains unclear. The combined activities of the 3- and 4--methyltransferase enzymes satisfactorily account for the pattern of SAM-dependent methylating activity shown by whole mycelia to phenolic substrates.


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