REGULATION OF BIOSYNTHESIS OF CARROT PHYTOALEXIN 6-METHOXYMELLEIN

6-Hydroxymellein (6HM) synthase, a biosynthetic enzyme of carrot phyto alexin 6-methoxymellein (6MM), is induced in root slices on treatment with elicitor. 6HM is synthesized via the polyketide pathway, and K-m values of the synthase for its substrates, acetyl-CoA and malonyl-CoA, were 15 and 6 mu M, respectively. The K-m values of the transacylases included in fatty acid synthesis which share these acyl-CoAs as commo n substrates were 4.7 and 3.2 mM. Another biosynthetic enzyme of 6MM p roduction, 6HM O-methyltransferase, also showed a very low K-m value f or S-adenosyl-L-methionine (0.4 mu M). These results suggest that 6MM synthetic enzymes have high affinities for acyl-CoAs and S-adenosyl-L- methionine and preferentially utilize these substrates as compared wit h other constitutive enzymes. The reaction products of 6HM synthase sh owed only slight inhibitory activities for 6HM synthase, while 6HM O-m ethyltransferase activity was significantly inhibited in the presence of either 6MM or S-adenosyl-L-homocysteine (K-i, 37 and 27 mu M, respe ctively). It appears, therefore that the product inhibition of 6HM O-m ethyltransferase is an important mechanism for regulating the concentr ation of 6MM in carrot cells.