Muscle‐specific expression of SRF‐related genes in the early embryo of Xenopus laevis.

We have isolated two members of the RSRF protein family, SL‐1 and SL‐2, in Xenopus laevis. Both proteins contain SRF‐type DNA binding domains and are related to the human protein, RSRFC4. SL‐1 constitutes a novel member of the RSRF family whilst SL‐2 is similar to human RSRFC4 throughout its length. SL‐1 protein recognizes the consensus DNA sequence CTA(A/T)4TAR in vitro and can bind to the same regulatory sites as other A/T‐rich sequence‐specific binding activities, such as the muscle‐specific regulatory factor, MEF‐2. Transcription of both Xenopus genes is restricted to the somitic mesoderm of early embryos and subsequently to the body muscle (myotomes) of the tadpole. In contrast, both genes are expressed ubiquitously in the adult frog. A binding activity, antigenically related to both human RSRFC4 and the SL‐2 gene product, is detected in Xenopus embryos and after gastrulation is localized to embryonic muscle. An indistinguishable binding activity is detected in many adult frog tissues. We conclude that the RSRF genes undergo a dramatic switch in their patterns of expression during development. We suggest that RSRF proteins may regulate muscle‐specific transcription in embryos, but acquire other roles during the course of development.