D-MEF2 - A DROSOPHILA MESODERM-SPECIFIC MADS BOX-CONTAINING GENE WITHA BIPHASIC EXPRESSION PROFILE DURING EMBRYOGENESIS

We have identified a mesoderm-specific Drosophila gene, designated D-m ef2. The encoded protein contains the MADS- and MEF2-specific domains, which are characteristic of the myocyte-specific enhancer factor 2 (M EF2) family of transcription factors. D-mef2 RNA is first detectable i n the presumptive mesoderm at late cellular blastoderm stage and is ex pressed in all mesoderm after invagination. Following the dorsal migra tion of the mesodermal layer, D-mef2 expression becomes restricted to the primordia for visceral muscle and the heart. In the second phase, D-mef2 expression is first distinct in heart precursors and then becom es prominent sequentially in visceral and somatic muscles. twi activit y is required for D-mef2 expression, while sna function may be needed for the maintenance of D-mef2 expression but not its initiation. D-mef expression is not dependent on the function of tin, and embryos that are deficient for the mesodermal gene DFR1 also show normal initiation of D-mef2 expression at blastoderm. These results suggest that D-mef2 could have a function in early mesoderm differentiation and may be re quired for subsequent cell fate specifications within the somatic and visceral/heart mesodermal layers.