A transgene target for positional regulators marks early rostrocaudal specification of myogenic lineages

In transgenic mice, muscle-specific regulatory elements from the myosin light chain (MLC) 1/3 locus drive graded expression of a linked CAT reporter gene in selected fast muscles along the anteroposterior axis of the adult animal. The gradient of MLC-CAT transcripts is established early in development, during the generation of somites from the paraxial mesoderm and the activation of myogenic factor gene expression, and is not reflected in the expression of the endogenous MLC1 gene. At later embryonic stages, the gradient of MLC-CAT transcripts persists in intercostal and intervertebral muscles, but is not maintained in other axial muscles. Profiles of CAT transgene activity reveal that the gradient is generated during the maturation of increasingly caudal somites, opposite to the direction of somite development, and is retained in dissociated somite cultures. We propose that coexpression of myogenic factors is necessary but not sufficient to regulate expression of the MLC-CAT transgene, which is responsive to additional positional cues in the embryo.