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

Abstract

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.

Xenopusmuscle development: From primary to secondary myogenesis

“…The involvement of E-boxes in mediating MRF activity in Xenopus is suggested by their presence in muscle-specific promoters (Su and Woodland, 1993;Gaillard et al, 1998). As observed in other species, strong evidence supports the notion that MRFs could cooperate with both ubiquitous bHLH proteins like E12 and E47 (Rashbass et al, 1992) and MEF2 proteins (Chambers et al, 1992) in the activation of musclespecific gene expression in Xenopus. During myotome development, XMRF4 mRNA is detected only at stage 18 (Jennings, 1992), suggesting that it is not involved in either muscle determination or early muscle differentiation, because many muscle gene products are already present before it is detectably expressed in primary muscle cells (Table 1).…”

Myocyte enhancer factor (MEF) 2C: a tissue-restricted member of the MEF-2 family of transcription factors.

“…The MEF-2C cDNA hybridized to a transcript of 6 kb that was detected at high levels in skeletal muscle and brain ( Fig. 2A), unlike other MEF-2 transcripts, which are widely expressed (10)(11)(12). A larger transcript was also detected at low levels in spleen and kidney.…”

“…Insight into the nature of the factors binding the MEF-2 site has recently been provided by the isolation of cDNAs encoding proteins that exhibit the DNA binding properties of MEF-2 (10)(11)(12). These proteins, originally termed RSRFs, for related to serum-response factor…”

“…The RSRF/MEF-2-like proteins that have been cloned thus far appear to represent the products of three separate genes, one of which gives rise to alternatively spliced transcripts (10)(11)(12). While these proteins show many properties expected of MEF-2, several aspects of their regulation are not easily explained.…”

Xenopus bagpipe‐related gene, koza, may play a role in regulation of cell proliferation

“…Based on these observations, additional transcription factors necessary for koza activity may be localized to kozaexpressing tissues. Such cofactors could include additional members of the MADS domain family, such as the Mef2 proteins, which are expressed in developing muscle tissue (Chambers et al, 1992;Edmondson et al, 1994) or SRF, which has been shown to physically interact with Nkx3-1 (Carson et al, 2000).…”