DIMER AS THE ACTIVE FORM OF A POLYKETIDE BIOSYNTHETIC ENZYME 6-HYDROXYMELLEIN SYNTHASE

Activity of 6-hydroxymellein synthase, an inducible polyketide biosynt hetic enzyme in carrot cell extracts, was appreciably inhibited in the presence of molar levels of NaCl or (NH4)(2)SO4. However, the salt-in duced inhibition of the synthase activity was reversible, and was almo st fully restored after the salts were removed. Highly purified 6-hydr oxymellein synthase showed essentially one band of M, 128 000 as analy sed-by SDS-PAGE. However, in gelfiltration analysis, the synthase acti vity was recovered in fractions corresponding to M, 285 000 when the e nzyme was eluted with buffered saline. By contrast, the enzyme protein was eluted at the position of M, 136 000 with loss of the activity un der high-salt condition. Activity of the M, 136 000 peptide was restor ed after desalting. The M, of the reactivated enzyme was found to shif t to 285 000. The results suggest that two polypeptides associate and form an active 6-hydroxymellein synthase.