Developmental dynamics of a polyhomeotic-EGFP fusion in vivo.

Polyhomeotic is a member of the Polycomb group of genes. The products of this group are chromatin-associated proteins that act together as multimeric complexes. These proteins are required for the maintenance of target gene repression in a permanent and heritable manner during development. In order to better understand the dynamics of their action during development, we generated 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 and its target specificity on polytene chromosomes. The distribution of the PH-EGFP protein is partly dependent on the presence of wildtype Polycomb protein, indicating that PH-EGFP behaves as does the wildtype PH protein. Therefore, the PH-EGFP chimera appears to be an appropriate reporter of PH protein distribution and a suitable tool for the study of Polycomb-group complex assembly in vivo. The subnuclear distribution of PH-EGFP is dynamic throughout 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 retraction through hatching of the larva, numerous discrete dots accumulate in the nucleus of epidermal cells. The increasing number of dots observed during development may indicate that PH-EGFP is recruited at different stages on different target sites, a result that is in good agreement with functional data previously reported.