The isolation and characterization of fatty-acid synthetase mRNA from rat mammary gland.

Abstract

The fatty acid synthetase complex of mammals is composed of two large and apparently identical subunits (Mr 240000), which contain the seven separate enzymatic activities, as well as the acyl carrier peptide, required for the synthesis of palmitate from acetyl-CoA and malonyl-CoA. In order to establish whether or not the multifunctional subunit of fatty acid synthetase is in fact synthesized as a single polypeptide chain, we have investigated the nature of the mRNA species coding for this protein. Total poly(A)-rich mRNA was isolated from lactating rat mammary gland: in this tissue, fatty acid synthetase is induced continuously throughout the lactation period and accumulates to as much as 10% of the total soluble protein, although the actual extent of induction as dependent on the litter size. Translation of the poly(A)-rich mRNA in a rabbit reticulocyte cell-free system produced, in addition to the major milk export proteins (α, β and γ-casein, and α-lactalbumin), it labeled polypeptide with the same electrophoretic mobility as the native fatty acid synthetase subunit. This translation product was specifically recognized by antisynthetase immunoglobulins and could be competed by excess native synthetase for antibody binding. A series of sucrose gradients was then employed to enrich the mRNA coding for fatty acid synthetase. The synthetase translational activity repeatedly exhibited a sedimentation coefficient of approximately 37 S, and co-purified with a peak of large RNA molecules located on the heavy side of the 28-S rRNA. Gel electrophoresis of the purified fatty acid synthetase mRNA fraction denatured with methylmercury hydroxide showed the presence of one major RNA species, whole molecular weight was determined to be about 3.5 × 106 (10000 nucleotides). This is approximately 3000 nucleotides greater than the minimum required to code for the entire rat fatty acid synthetase subunit. The identification, therefore, of this mRNA species which directs the translation of the complete synthetase subunit in vitro, demonstrates that the native fatty acid synthetase monomer does arise as a single polypeptide chain synthesized front one contiguous mRNA, and confirms the truly multifunctional nature of this protein.