Alternative usage of different poly(A) addition signals for two major species of mRNA encoding human aromatase P-450

Two cDNA clones for human placental aromatase P-450 (P-450AROM) have been isolated and sequenced. The insert of one clone (2894 bp) contains an open reading frame encoding a protein consisting of 503 amino acid residues together with a 49 bp 5'-untranslated stretch and a 1336 bp 3'-noncoding region to which a poly(A) tract is attached. Three potential poly(A) addition signals are detected in this 3'-noncoding region. The other clone contains a shorter cDNA insert, the nucleotide sequence of which overlaps with most of the sequence of the longer cDNA insert (nucleotides 36-2355) except for one nucleotide substitution. The 3'-noncoding region of this shorter cDNA is only 846 bp in length but a poly(A) tract is also attached to its 3'-terminus. Northern blot analysis of human placental RNA reveals the presence of two major mRNA species of 3.4 and 2.9 kb when probes excised from the overlapping region of these two cDNAs are employed. The 2.9 kb mRNA is not detected, however, when a fragment of the non-overlapping region of the longer cDNA is used as a probe. It is therefore concluded that the two major species of P-450AROM mRNA are formed as a consequence of alternative processing of precursor mRNA(s).