Rabbit muscle creatine kinase: genomic cloning, sequencing, and analysis of upstream sequences important for expression in myocytes.
Muscle creatine kinase (MCK) is a major enzyme of cellular energy metabolism that is expressed upon differentiation of myoblasts into myotubes. Previously we cloned and sequenced the entire rabbit enzyme cDNA which was used as a probe in these studies to obtain a genomic clone from a rabbit library. The transcription start site was identified by primer extension analysis and over 800 bp of 5' flanking DNA was sequenced. Comparison of this sequence with the published sequences from the upstream regions of the mouse MCK gene and the human MCK gene showed two conserved regions and a large intervening block of non-conserved sequence. The conserved regions are separated by about 800 bp in the mouse and by about 400 bp in the human, but are much closer (200 bp) in the rabbit. The upstream conserved region of the mouse gene encompasses a region possessing the properties of an enhancer and containing two MyoD binding sites; the downstream element is adjacent to the start of transcription. A set of of overlapping deletions of the 5' upstream DNA was fused to the CAT gene and transfected into mouse C2 myocytes, chick primary myocytes, and chick primary liver cells. Constructs which contained both conserved 5' regions were strongly expressed in C2 and chick myocytes, but were not expressed (above background) in primary liver cells. Surprisingly, while the upstream enhancer element was required for strong expression in C2 myocytes, it was less important for expression in chick myocytes. This suggests that there are important muscle-specific transcriptional signals in the proximal promoter region of mammalian MCK genes.