Choice of 3' cleavage/polyadenylation site in beta-tropomyosin RNA processing is differentiation-dependent in mouse BC3H1 muscle cells.

The rodent beta-tropomyosin (TM) gene produces either a 1.2-kilobase (kb) skeletal muscle beta-TM mRNA or a 1.1-kb fibroblast/smooth muscle TM-1 mRNA through tissue-specific alternative exon splicing and 3' cleavage/polyadenylation at two alternative poly(A) sites. beta-TM mRNA contains exon 6b, 9a, and the poly(A) site immediately following exon 9a, whereas TM-1 mRNA contains exon 6a, 9b, and the poly(A) site following exon 9b. We isolated a novel 2.1-kb beta-TM cDNA clone, pUTM, from a cDNA library of 2-day differentiated mouse BC3H1 muscle-like cells. This cDNA contains the entire sequence of mature beta-TM mRNA with a normal but unused poly(A) site associated with exon 9a. Instead, 3' cleavage/polyadenylation of this cDNA occurred at the exon 9b-associated distal poly(A) site, resulting in the retention of a 1-kb intron and the TM-1 exon 9b. We identified a 2.3-kb functional mRNA, UTM RNA, corresponding to pUTM. UTM RNA appeared early during BC3H1 cell differentiation and gradually decreased as the beta-TM mRNA increased. UTM RNA was also detected in mouse C2C12 muscle cells and in skeletal muscle tissue isolated from mouse leg. Thus, in the processing of beta-TM gene transcripts, selection of alternative terminal exons and alternative poly(A) sites are not necessarily linked as they appear to be in other gene systems.