Molecular and quantitative characterisation of the porcine embryonic myosin heavy chain gene

The porcine embryonic myosin heavy chain (MyHC) is a major isoform in foeta l skeletal muscle, and is the last remaining major porcine skeletal MyHC ge ne to be isolated and characterised. We report here on its cDNA and genomic isolation, molecular characterisation, quantification and expression. Unli ke all other porcine and mammalian skeletal MyHC genes reported to date, th e deduced translated start site of the porcine embryonic gene was located i n exon 2, instead of exon 3. Its promoter conferred differentiation-specifi c expression. We found, by quantitative real-time RT-PCR, that for much of gestation the embryonic MyHC was by far the most transcriptionally active g ene compared with the slow/I and perinatal MyHC isoforms, and it was consis tently more highly expressed than the perinatal isoform throughout gestatio n. The embryonic MyHC isoform was, however, rapidly down-regulated at aroun d birth. By contrast, 22 weeks after birth, the porcine perinatal isoform r emained detectable by PCR. Additionally, we discovered the presence of diff erential splicing at the 3'-end of the embryonic MyHC gene that resulted in an in-frame deletion, with the consequential loss of 93 amino acids close to the ACD domain, a region that is important for the assembly of myosin fi laments. The detection of this truncated variant points to a possible major post-transcriptional mechanism of embryonic MyHC regulation that may be li nked to myosin filament formation or turnover.