The human embryonic myosin alkali light chain gene: use of alternative promoters and 3′ non-coding regions

Recently we have found evidence that the human embryonic myosin alkali light chain (MLC1 emb) gene has two functional promoters and that its mRNAs exhibit heterogeneity in their 3'untranslated regions (UTR). To study this more in detail we have isolated and characterized the human MLC1emb gene. We focussed in particular on 2 kilobases of 5'flanking region and the alternative 3'UTRs. RNA primer extension and S1 mapping analyses revealed that the MLC1emb gene can indeed be driven either by a proximal or a distal promoter, both in fetal and adult cardiac tissue. These MLC1emb RNAs can contain either the proximal or distal 3'UTR. In contrast to this, in fetal as well as adult masseter muscle MLC1emb mRNA is predominantly transcribed from the proximal promoter and contains mainly the distal 3'UTR. These results explain the known heterogeneity of MLC1emb mRNAs. Finally, we present evidence that the murine MLC1emb gene also contains a functional distal promoter element which has hitherto been undetected.