Developmental dynamics of a polyhomeotic-EGFP fusion in vivo

polyhomeotic is a member of the Polycomb group of genes. The products of th is group are chromatin-associated proteins that act together as multimeric complexes. These proteins are required for the maintenance of target gene r epression in a permanent and heritable manner during development. In order to better understand the dynamics of their action during development, we ge nerated transgenic flies expressing a polyhomeotic protein tagged with the enhanced green fluorescent protein. Here we show that this fusion protein ( PH-EGFP) retains both the functional properties of the endogenous protein a nd its target specificity on polytene chromosomes. The distribution of the PH-EGFP protein is partly dependent on the presence of wildtype Polycomb pr otein, indicating that PH-EGFP behaves as does the wildtype PH protein. The refore, the PH-EGFP chimera appears to be an appropriate reporter of PH pro tein distribution and a suitable tool for the study of Polycomb-group compl ex assembly in vivo. The subnuclear distribution of PH-EGFP is dynamic thro ughout development. In the interphase nucleus at the cellular blastoderm, a diffuse granular pattern is observed. From the early gastrula stage onward , a few brighter dots appear. As development progressed from germ band retr action through hatching of the larva, numerous discrete dots accumulate in the nucleus of epidermal cells. The increasing number of dots observed duri ng development may indicate that PH-EGFP is recruited at different stages o n different target sites, a result that is in good agreement with functiona l data previously reported.