The enzyme aromatase P-450 (P450arom) catalyses the conversion of androgen to oestrogen. A cDNA insert encoding P450arom was isolated from a rainbow trout (Oncorhynchus mykiss) ovary cDNA library. The insert was sequenced and found to contain an open-reading frame predicted to encode a protein of 522 amino acid residues. The deduced polypeptide is 52% homologous with human, mouse and rat P450arom and 53% homologous with that of chicken. The insert was confirmed to encode P450arom by introducing it into COS-1 monkey kidney tumour cells (COS-1 cells) and detecting the conversion of testosterone to oestradiol-17β by radioimmunoassay. The N-terminal region of the deduced polypeptide was 19 amino acids longer than that of the other four species, and was found by hydropathy plotting to be very hydrophobic.
Northern blot analysis revealed 2.6kb RNA transcripts which were present in the trout ovary during vitellogenesis and hybridized to the cDNA insert. In preparations from subsequent stages of ovarian development, no RNA transcripts hybridized to the probe. Since the RNA transcripts are present only during the stage of oestradiol-β production by the ovarian follicles, oestradiol-17β production may be regulated, in part, by the amount of P450arom mRNA present.