A major question regarding vertebrate heart development is how transcription factors broadly expressed in the embryo regulate the tightly restricted expression patterns of genes involved in cardiac muscle differentiation. Latinkić and co-workers() address this by examining how transcription factors interact to regulate the pan-myocardial expression of the Xenopus gene myosin light chain 2 (XMLC2),a sensitive marker for the onset of cardiac muscle differentiation. They show that XMLC2 has a remarkably small 82 bp core promoter containing several transcription factor-binding sites: two GATA-binding sites and a low-affinity serum response factor (SRF)-binding site overlapping a YY1-binding site. Although GATA4, SRF and YY1 are synthesised broadly in the developing embryo, it is their overlapping expression in the heart that apparently provides cardiac-specific expression of XMCL2. The XMCL2 promoter also drives pan-myocardial expression of a reporter gene in mice and the authors go on to consider the evolution of the mechanisms that drive XMCL2 expression.
