radioactive acetate. This procedure should cause the dilution of radioactive malonyl-CoA formed from radioactive acetate. (Also, the added malonate might alter the acetate-malonate equilibrium to prevent incorporation of radioactive label into malonyl-CoA.) If no labeled acetate were converted to malonyl-CoA, 100% of the rubrofusarin molecule's radioactivity would be in the acetate of the terminal starting unit.
Flooding the cultures with nonradioactive malonate increased the amount of radioactivity of the carbon in Position 2 of the ring system of rubrofusarin; this supports the view that acetate serves as the terminal starting unit in the formation of a polyketide chain.
Since the increase in percent incorporation was 47.5 % instead of the theoretical 100 x, apparently, some radioactive acetate was converted to malonyl-CoA, decreasing the percentage value of the experimental results. Addition of nonlabeled malonate, however, caused a significant increase in radioactivity of carbon in Position 2 of rubrofusarin by 133 % over the results of the acetate only experiment.
Schmidt degradation of acetate recovered from the Kuhn-Roth oxidation of radioactive rubrofusarin revealed no randomization of radioactive label in the terminal starting acetate unit. Absence of such randomization indicated that the original radioactive acetate had not been metabolized to another compound before incorporation into rubrofusarin REFERENCES