Evidence for a cytosolic precursor of chick embryo liver mitochondrial delta-aminolevulinate synthase

Following the recent demonstration [Borthwick, I.A., Srivastava, G., Brooker, J.D., May, B.K. and Elliott, W.H. (1982) Eur. J. Biochem. in press] that chick embryo liver mitochondrial delta-aminolevulinate synthase has a minimum molecular weight of 68,000 (rather than the hitherto accepted value of 49,000), we have shown that the primary translation product of delta-aminolevulinate synthase mRNA is a protein of molecular weight 74,000. This protein has for the first time been shown to occur in the cytosol fraction of drug-treated chick embryo livers. This form does not occur in mitochondria nor does the smaller mitochondrial form occur in the cytosol. It is concluded that the 74,000 molecular weight protein is a precursor which is processed during transport into the mitochondria. In vivo labelling experiments are consistent with this conclusion.