In vivo observation of the puff-specific protein no-on transient A (NONA) in nuclei of Drosophila embryos.

The spatial distribution of no-on transient A (NONA), a protein associated with specific puffs on polytene chromosomes, was followed in nuclei of living Drosophila embryos by microinjection of fluorescently labeled monoclonal antibody to NONA. The injected antibodies remained active until the larval stage, revealing the distribution of the NONA protein throughout embryogenesis. Most injected animals completed embryonic development and hatched as normal larvae. NONA was restricted to the cytoplasm until the end of cycle 11. We document an active uptake of the NONA-antibody complex into early interphase nuclei from nuclear cycle 14 onwards, following each mitosis. Significant differences in the distribution of the protein between fixed and living embryos were apparent, particularly at high resolution. The NONA protein was localized in the nuclei of living embryos at discrete sites, most of which lay at the periphery and some of which were tightly clustered. The constellation of sites changed with time; in some nuclei these changes were fast whereas in other nuclei the pattern was quite stable. These data suggest that specific protein complexes associated with active interphase chromatin, and possibly chromatin in general, are mobile in the living organism.