A putative protein–RNA complex regulates posttranscriptional processing of cytochrome P450 aromatase (CYP19A1) in bovine granulosa cells

Follicle growth and granulosa cell health are dependent on the secretion of estradiol from granulosa cells. Estradiol is synthesized from androgen precursor by cytochrome P450 aromatase (CYP19A1), and in cattle CYP19A1 messenger RNA has a short half‐life but a long (3.5 kb) 3′‐untranslated region (3′UTR), suggesting that posttranscriptional regulation may be important for control of enzyme activity. We tested this hypothesis by inserting the CYP19A1 3′UTR and fragments thereof into a reporter vector between the end of the luciferase coding region and the polyadenylation signal. The full‐length aromatase 3′UTR suppressed luciferase activity to 10% of control levels, and smaller fragments showed that this inhibitory activity lies between +926 and +1134 of the 3′UTR. Protein–RNA cross‐linking experiments revealed that these 3′UTR fragments formed an RNA–protein complex of approximately 70 kDa that was present in granulosa cells but not in corpus luteum, lung, liver, kidney, pancreas, or bladder extracts. The RNA‐binding activity was specific to the 3′UTR, as shown by competition experiments with unlabeled RNA, and was present only in 3′UTR constructs that inhibited luciferase activity. These data suggest that posttranscriptional regulation is an important component of the control of CYP19A1 expression and involves protein binding to a specific sequence in the 3′UTR. The bovine CYP19A1 (aromatase) messenger RNA (mRNA) has a short half‐life and possesses a long 3′‐untranslated region (3′UTR). We identified a short sequence within this 3′UTR that forms a complex with granulosa cell proteins and which reduces the translation of a reporter gene. This sequence may explain the short half‐life of this mRNA.