Regulation of expression of a Xenopus borealis embryonic/larval α3 skeletal‐actin gene

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Abstract
We have isolated a genomic clone, related in sequence to the skeletal-actin gene sub-family. It is expressed in the skeletal muscle of embryos from the neurula stage onwards and in tadpoles, but not in adults. The equivalent Xenopus laevis gene is expressed as a major transcript in adult muscle, as well as at earlier stages. The intron/exon structure is typical of vertebrate skeletal-actin genes, as is the possession of multiple copies of three serum-response elements in the promoter of this gene. The Xenopus actin and beta-globin genes were fused in their second introns. This construct, which contained 2.4 kb of upstream sequence, was injected into fertilized eggs at the two-cell stage. It showed the normal pattern of tissue-specific transcription. Thus all of the information necessary for appropriate expression of this actin gene in the embryo is contained in the region that extends from a point 2.4 kb upstream of transcription initiation to the centre of the second exon. A series of enhancer constructs were made in which upstream regions of the actin gene were placed upstream of a X. laevis beta-globin gene. The region immediately adjacent to the promoter, containing the three serum-response elements, was able to drive muscle-specific expression, and there was also a general enhancement of transcription by regions further upstream.