Intranuclear targeting of DNA replication factors

Mammalian nuclei are highly organized into functional compartments. Major n uclear processes like DNA replication and RNA processing take place in dist inct foci. These microscopically visible foci are formed by the assembly of , for example, DNA replication factors and associated proteins into megadal ton complexes often referred to as protein machines or factories. Thus far, two proteins, DNA ligase I and DNA methyltransferase (DNA MTase), have bee n analyzed in greater detail. In both cases, the assembly process appears t o be controlled by distinct targeting sequences that were attached to the c atalytic protein core in the course of evolution and mediate the associatio n with replication factories in mammalian cells. The dynamics of these nucl ear structures throughout the cell cycle are analyzed using green fluoresce nt protein (GFP). Further studies are needed to elucidate the architecture, regulation, and role of these subnuclear structures. (C) 1998 Wiley-Liss, Inc.