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 h ave been isolated from a stage 42 cDNA library. Sequence analysis reve als that the amphibian MLClf and MLC3f isoforms are similar to the mam malian acid avian cognates. The two isoforms share a common 141-amino- acid carboxy-terminal region but differ in their specific N-terminal r egions. These are 49 and 9 residues long for the MLC1f and MLC3f isofo rms, respectively. This suggests a genomic organization similar to the mammalian and avian genes, with two promoters and alternative splicin g. The developmental expression of the MLC1f/3f mRNAs was studied by N orthern blot and RNase protection and their spatial expression analyze d by in situ hybridization. Both the MLClf and MLC3f mRNAs can be dete cted in the developing embryo from the end of gastrulation and accumul ate rapidly in the semitic mesoderm. Expression of the MLC1f/3f gene c an 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 MLC 1f/3f gene can be detected in embryonic or adult cardiac muscle, their expression being restricted to semitic muscle. Together, these data d emonstrate that expression of the MLC1f/3f gene provides a sensitive a nd specific marker for skeletal muscle differentiation. Ectopic expres sion 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. (C) 1995 Academic Press, Inc.