Molecular cloning of a cDNA showing alternative splicing of the 5′‐untranslated sequence of mRNA for human aromatase P‐450

Abstract

A new type of full‐length cDNA clone encoding human aromatase P‐450 was isolated from a human placental cDNA library. The clone, designated as pES‐4, has a 3130‐bp insert. The nucleotide sequences of the translated region and the 3′‐untranslated region of the insert of pES‐4 are exactly identical with those of the cDNA clone characterized previously. However, the sequence of the 5′‐untranslated region of the insert has characteristic feature, i.e. an extra sequence of 109 bp is present at a junction between exon 1 and exon 2 on the processed human aromatase mRNA. Analysis of the genomic clones containing the region between exon 1 and exon 2 of the human aromatase P‐450 gene reveals that the 109‐bp genomic segment, encoding the same sequence as the extra sequence observed in pES‐4, is located approximately 10‐kbp downstream of exon 1 and that the nucleotide sequences of the 5′‐flanking and the 3′‐flanking regions of the segment conform to the GT‐AG rule for RNA splicing. By means of reverse transcription and polymerase chain reaction, relative amounts of the pES‐4‐type mRNA are estimated to be approximately 4.8% and 2.3% of the processed aromatase P‐450 mRNA in human placenta and human BeWo choriocarcinoma cells, respectively. These results indicate that the segment of 109 bp between exon 1 and exon 2 is a new exon hitherto unidentified and that heterogeneity observed in the 5′‐untranslated sequence of human aromatase P‐450 mRNA is, at least in part, caused by alternative splicing of this new exon. Copyright © 1993, Wiley Blackwell. All rights reserved