All three promoters of the acetyl-coenzyme A-carboxylase alpha-encoding gene are expressed in mammary epithelial cells of ruminants.

The activity of the enzyme acetyl-CoA-carboxylase alpha (ACC-alpha) is rate limiting for the de novo synthesis of fatty acids. The encoding gene is expressed from three promoters in ruminants (PI-PIII). Their individual contribution to the formation of milk fat is unknown. Promoter-specific molecular probes were hybridized in situ to serial sections of mammary glands from cows and sheep to determine their developmental and spatial expression profile in the udder. We show that all three promoters are active in mammary epithelial cells (MECs) of udders from both species. This implies that, in principle, none of these promoters can be singled out as the key element controlling the ACC-alpha-related contribution to establishment of milk fat content, although the activity of PIII only is known to be disproportionally stimulated by lactation in MECs. We propose that all three promoters may be relevant for milk fat synthesis in cattle, whereas PII and PIII are crucial for milk fat formation in sheep. We show also that ACC-alpha synthesis is not strictly coupled to casein synthesis, particularly during pregnancy and involution.