Localization of bacterial DNA polymerase: evidence for a factory model of replication

Differentiation-related mechanisms which suppress DNA replication

Differentiation of mammalian cells implies cessation of DNA replication and cell proliferation; the potential controls of this coupling are examined here. It is clear that the known or proposed mechanisms of down-regulation of replicative cellular activities vary in different lineages of cell differentiation, and occur in all phases of the cell cycle. In G1 these regulators include p21/Cip1 or p27/Kip1, pRb, and p53; the novel, recently reported mechanisms of their action are summarized. In S phase the availability of nucleotide precursors, the origin recognition complex (ORC), and other replication proteins may be important in differentiation, and in G2 phase the cdc2/cyclin B complex and replication licensing factors determine normal G2 traverse versus an arrest or polyploidisation. Other replication-related mechanisms include transcription factors, e.g., Sp1, telomerase, and nuclear matrix changes. Thus, differentiation alters the activity not only of the various checkpoint proteins, but also of the components of the replicative machinery itself.

The assembly and migration of SeqA-Gfp fusion in living cells of Escherichia coli

SeqA protein, which binds to hemi-methylated GATC sequences of DNA, is localized to discrete fluorescent foci in wild-type Escherichia coli cells. In this work, we observed cellular localization of the SeqA-Gfp fusion in living cells. SeqA-Gfp was localized to a discrete focus or foci in wild-type and seqA null mutant cells, but the fusion was dispersed in the whole cell in dam null mutant cells lacking Dam methyltransferase. These results were consistent with the previous description of the localization of SeqA by immunofluorescence microscopy. Time-lapse experiments revealed that duplicated SeqA-Gfp foci migrated rapidly in opposite directions. Flow cytometry demonstrated that the fusion restored synchronous replication of chromosomal DNA from multiple origins in seqA null mutant cells, indicating that SeqA-Gfp is biologically active. Immunoprecipitation of the fusion from cell extracts using anti-Gfp antibody indicated that the fusion was assembled with the wild-type SeqA protein.