Similar types of study in an early industrial strain of S.
cinnamonensis C730.1 have indicated that crotonyl-CoA
reductase (CCR), which catalyses the conversion of
crotonyl-CoA to butyryl-CoA, plays an important role in
providing ethylmalonyl-CoA for monensin biosynthesis,
but not methylmalonyl-CoA (Liu & Reynolds, 1999). Two
major monensin products from fermentations of S.
cinnamonensis are monensin A and monensin B (Fig. 2a).
Monensin A is made using an ethylmalonyl-CoA instead of
methylmalonyl-CoA during the fifth elongation step (Liu
& Reynolds, 1999). We have previously cloned and sequenced
S. cinnamonensis ccr and showed that insertional
inactivation of this gene in the C730.1 strain generates an