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

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 β‐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 β‐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.