Abstract
To prevent the transmission of damaged genomic material between generations, cells require a system for accommodating DNA repair within their cell cycles. We have previously shown that Escherichia coli cells subject to a single, repairable site-specific DNA double-strand break (DSB) per DNA replication cycle reach a new average cell length, with a negligible effect on population growth rate. We show here that this new cell size distribution is caused by a DSB repair-dependent delay in completion of cell division. This delay occurs despite unperturbed cell size regulated initiation of both chromosomal DNA replication and cell division. Furthermore, despite DSB repair altering the profile of DNA replication across the genome, the time required to complete chromosomal duplication is invariant. The delay in completion of cell division is accompanied by a DSB repair-dependent delay in individualization of sister nucleoids. We suggest that DSB repair events create inter-sister connections that persist until those chromosomes are separated by a closing septum.
Highlights
The presence of a 246bp interrupted DNA palindrome inserted at the lacZ locus of an otherwise wild-type Escherichia coli chromosome results in a chronic replication-dependent DNA double-strand break (DSB) that is efficiently repaired by homologous recombination with an unbroken sister chromosome [1]
We have previously shown that Escherichia coli cells subject to a single, repairable site-specific DNA doublestrand break (DSB) per DNA replication cycle reach a new average cell length, with a negligible effect on population growth rate
The bacterium Escherichia coli has a remarkable cell cycle where overlapping rounds of DNA replication can occur in a single generation between cell birth and division
Summary
The presence of a 246bp interrupted DNA palindrome inserted at the lacZ locus of an otherwise wild-type Escherichia coli chromosome results in a chronic replication-dependent DNA double-strand break (DSB) that is efficiently repaired by homologous recombination with an unbroken sister chromosome [1] This DSB is caused by the Mre11/Rad related endonuclease SbcCD cleaving a hairpin structure formed by the palindrome on one of a pair of sister chromosomes during DNA replication (Fig 1A). The observed increase in cell length is primarily due to an increase in unit cell size that is independent of the well-described DNA damage-induced inhibitor of cell division SfiA [2] This DSB acts as a defined perturbation of the cell cycle that maintains growth rate while altering cell size. It is biologically relevant, with efficiently repaired replication-dependent DSBs estimated to occur in approximately 1 in every 5 normal E. coli cell division cycles [3] while palindrome-induced breaks occur at approximately 100% of replication cycles and 100% of cell division cycles [4]
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