The identification of elements determining the different DNA binding specificities of the MADS box proteins p67SRF and RSRFC4.

The human transcription factors p67SRF and RSRFC4 recognise similar but distinct binding sites which are found in the promoters of both muscle-specific and 'immediate early' genes. Both proteins share a common basic DNA-binding domain, which is defined by the MADS box homology region. The DNA-binding specificity of a truncated form of p67SRF (coreSRF) can be converted to that of RSRFC4. Removal of residues immediately N-terminal to the MADS box relaxes the specificity of coreSRF for its cognate sequence (CC(A/T)6GG) as it improves binding to the RSRFC4 site (CTA(A/T)4TAG). Moreover, the introduction of a single, additional mutation, K154E, into the N-terminal truncated derivative completes the change in specificity to the RSRFC4 binding site. It also influences the salt dependence of DNA binding and ternary complex formation with p62TCF. However, residues at this position do not appear to be involved in direct basepair recognition. These results indicate that although the DNA binding specificity of p67SRF can be converted to that of RSRFC4, the two proteins may bind DNA in different ways. Furthermore, they suggest that binding site specificity can be determined by an indirect mechanism involving residues which are not directly involved in base recognition.