The MLC1f/3f Gene Is an Early Marker of Somitic Muscle Differentiation in Xenopus laevis Embryo

cDNAs clones encoding the MLC1f and MLC3f proteins of Xenopus laevis have been isolated from a stage 42 cDNA library. Sequence analysis reveals that the amphibian MLC1f and MLC3f isoforms are similar to the mammalian and avian cognates. The two isoforms share a common 141-amino-acid carboxy-terminal region but differ in their specific N-terminal regions. These are 49 and 9 residues long for the MLC1f and MLC3f isoforms, respectively. This suggests a genomic organization similar to the mammalian and avian genes, with two promoters and alternative splicing. The developmental expression of the MLC1f/3f mRNAs was studied by Northern blot and RNase protection and their spatial expression analyzed by in situ hybridization. Both the MLC1f and MLC3f mRNAs can be detected in the developing embryo from the end of gastrulation and accumulate rapidly in the somitic mesoderm. Expression of the MLC1f/3f gene can also be detected in animal cap explants which have been induced to form mesodermal derivatives by exposure to activin A or bFGF. However, unlike other muscle-specific markers, neither transcript from the MLC1f/3f gene can be detected in embryonic or adult cardiac muscle, their expression being restricted to somitic muscle. Together, these data demonstrate that expression of the MLC1f/3f gene provides a sensitive and specific marker for skeletal muscle differentiation. Ectopic expression of myogenic factors in animal caps induces the expression of the MLC1f/3f gene, suggesting that the amphibian gene, like its mammalian and avian counterparts, is a regulatory target for members of the MyoD family of transcription factors.