Unlike eukaryotic cells, there are no well-defined microtubule-based spindle structures for segregating homologous chromosomes during bacterial cell division. The nature of the machinery for ensuring that newly duplicated chromosomes are equally distributed to daughter cells is also obscure for prokaryotes. Recently, two papers address these questions by providing direct evidences that the newly replicated chromosomes are aligned with their origins oriented toward the pores of the predivisional cell. In the bacterium B. subtilis, a tandem copies of the lactose operon operator was introduced into the chromosome near the replication origin or terminus. The position of the operator cassettes was visualized by LacI repressor fused to the green fluorescent protein. In sporulating bacteria, which undergo asymmetric cell division, origins localized near each pore of the cell whereas termini were located at middle of the cell. In growing cells, which undergo binary fission, origins were observed at various positions but preferentially toward the pores in early cell cycle stage. However, termini showed little preference for the pores. In addition, the cellular homologs of plasmid partitioning (par) proteins, ParA and ParB localize to both poles of the predivisional cell following the completion of DNA replication. ParB specifically binds to DNA sequences adjacent to the origin of replication. Overexpression of ParA and ParB in Caulobacter crescentus disturbs polar localization and results in defects in both cell division and chromosome partitioning. These results suggest that ParA and ParB, the components of a bacterial mitotic-like apparatus, are involved in moving the newly replicated chromosomes toward the pores of the cell.
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