Identification of the functional promoter regions in the human gene encoding the myosin alkali light chains MLC1 and MLC3 of fast skeletal muscle.

The human gene encoding the alkali myosin light chains (MLC) 1 and 3 of fast skeletal muscle has been isolated. Two separate start sites for transcription have been identified by S1 analysis of muscle RNA. The nucleotide sequences of both proximal promoter regions have been determined and compared to the corresponding gene regions of other species. Several conserved promoter elements were located within 140 nucleotides upstream of the mRNA cap site, whereas further upstream no homologous sequences were found. Unidirectional 5' deletion mutants of both MLC promoters were used to direct bacterial chloramphenicol acetyltransferase activity in transient transfection assays of muscle and nonmuscle cells. Approximately 120 nucleotides of the MLC1 promoter and 80 nucleotides of the MLC3 promoter were sufficient for the transcriptional activation in primary myotubes and to a lower degree also in fibroblasts and hepatocytes. The preferential expression in muscle cells was not dependent on the conserved MLC consensus sequence, CCTTTTATAG, but it absolutely required the CCAT box or the CAT-like box in the MLC1 and MLC3 promoters, respectively. The weak activity of the MLC1 promoter was markedly enhanced in myotubes when DNA from the 3' gene flanking sequence was included in the chloramphenicol acetyltransferase constructs.