FtsK controls metastable recombination provoked by an extra Ter site in the Escherichia coli chromosome terminus

Abstract

The FtsK protein is required for septum formation in Escherichia coli and as a DNA translocase for chromosome processing while the septum closes. Its domain of action on the chromosome overlaps the replication terminus region, which lies between replication pause sites TerA and TerC. An extra Ter site, PsrA*, has been inserted at a position common to the FtsK and terminus domains. It is well tolerated, although it compels replication forks travelling clockwise from oriC to stall and await arrival of counter-clockwise forks. Elevated recombination has been detected at the stalled fork. Analysis of PsrA*-induced homologous recombination by an excision test revealed unique features. (i) rates of excision near PsrA* may fluctuate widely from clone to clone, a phenomenon we term whimsicality, (ii) excision rates are nevertheless conserved for many generations, a phenomenon we term memorization; their metastability at the clone level is explainable by frequent shifting between three cellular states--high, medium and low probability of excision, (iii) PsrA*-induced excision is RecBC-independent and is strongly counteracted by FtsK, which in addition is involved in its whimsicality and (iv) whimsicality disappears as the distance from the pause site increases. Action of FtsK at a replication fork was unexpected because the factor was thought to act on the chromosome only at septation, i.e. after replication is completed. Idiosyncrasy of PsrA*-induced recombination is discussed with respect to possible intermingling of replication, repair and post-replication steps of bacterial chromosome processing during the cell cycle.