The expression of a "Xenopus borealis" cardiac actin gene in normal and transformed frog embryos

The major aim of this project has been to ascertain if the expression of a cloned Xenopus gene, which is normally expressed in a tissue - specific fashion during early development, is regulated in the same way as its chromosomal counterpart in micro- injected Xenopus embryos. A number of clones, which contain genes encoding different actin isoforms, have been isolated from Xenopus borealis genomic libraries. One of these contains an entire cardiac actin gene, on the basis of the isotype - specific sequence of its encoded product. Indeed, in the adult frog, the chromosomal gene is only expressed in the heart. However, in the embryo, transcripts are also detected in the myotomes, which contain skeletal muscle cells. Two transcriptional assays have been developed, so that transcripts from the unmodified, cloned X. borealis cardiac actin gene can be detected separately from endogenous transcripts in micro-injected X. laevis embryos. In such transformed animals, injected linear DNA forms high-molecular-weight extrachromosomal concatenates, which replicate and become relatively evenly distributed throughout all tissues. Properly initiated transcripts from the cloned gene are correctly localised to the myotomes in both neurulae and tadpoles. The temporal regulation of expression also shares strong similarities with that of the endogenous, chromosomal actin gene. In a preliminary investigation of the sequences responsible for this regulation, a fusion construct between the first two actin exons and the last exon of a mouse β-globin gene has been injected. The same wide distribution of DNA, but spatially restricted pattern of expression, as the actin gene is found, whereas transcripts from a histone-globin fusion gene are formed in all tissues. This is the first report of correct spatial control of expression from an injected, cloned gene in Xenopus. I discuss the wider significance of these results for future studies on the early developmental regulation of gene expression