A novel flavonol-specific sulfotransferase was partially purified from the shoot tips of Flaveria bidentis var. Angustifolia O.K. (Asteraceae) by chromatography on 3'-phosphoadenosine 5'-phosphate-agarose affinity column and chromatofocusing on Mono P. The latter step resulted in the separation of two isoforms, both of which exhibited expressed specificity for position 7 of quercetin 3,3'- and quercetin 3,4'-disulfate. The 7-sulfotransferase isoforms I and II had a pH optimum of 7.5 in phosphate buffer, apparent pl values of 6.5 and 6.3, and an M(r) of 35,000. They had no requirement for divalent cations and were not inhibited by EDTA or SH group reagents. Their K(m) values for both the sulfate donor and flavonol acceptor were of the same order of magnitude (0.20-0.46 micromolar). This enzyme, together with the recently reported flavonol 3-, 3'-, and 4'-sulfotransferases from F. chloraefolia (L Varin, RK Ibrahim [1989] Plant Physiol 90: 977-981) form the complement involved in the biosynthesis of polysulfated flavonols in this genus. A proposed sequential order for the enzymatic sulfation in both species is described.