It is demonstrated that crotonyl-CoA reductase (CCR) plays a significant role in providing methylmalonyl-CoA for monensin biosynthesis in oil-based 10-day fermentations of . Under these conditions L1, a derivative of a high-titre producing industrial strain C730.1 in which has been insertionally inactivated, produces only 15 % of the monensin yield. Labelling of the coenzyme A pools using [H]--alanine and analysis of intracellular acyl-CoAs in the L1 and C730.1 strains demonstrated that loss of led to lower levels of the monensin precursor methymalonyl-CoA, relative to coenzyme A. Expression of a heterologous gene from fully restored monensin production to the L1 mutant. Using C730.1 and an oil-based extended fermentation an exceptionally efficient and comparably intact incorporation of ethyl [3,4-C]acetoacetate into both the ethylmalonyl-CoA- and methylmalonyl-CoA-derived positions of monensin was observed. No labelling of the malonyl-CoA-derived positions was observed. The opposite result was observed when the incorporation study was carried out with the L1 strain, demonstrating that insertional inactivation has led to a reversal of carbon flux from an acetoacetyl-CoA intermediate. These results dramatically contrast similar analyses of the L1 mutant in glucose-soybean medium which indicate a role in providing ethylmalonyl-CoA but not methylmalonyl-CoA, thus causing a change in the ratio of monensin A and monensin B analogues, but not the overall monensin titre. These results demonstrate that the relative contributions of different pathways and enzymes to providing polyketide precursors are thus dependent upon the fermentation conditions. Furthermore, the generally accepted pathways for providing methylmalonyl-CoA for polyketide production may not be significant for the high-titre monensin producer in oil-based extended fermentations. An alternative pathway, leading from the fatty acid catabolite acetyl-CoA, via the CCR-catalysed reaction is proposed.


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