To let go or not to let go: how ParA can impact the release of the chromosomal anchoring in Caulobacter crescentus

Abstract

Chromosomal maintenance is vital for the survival of bacteria. In Caulobacter crescentus, chromosome replication initiates at ori and segregation is delayed until the nearby centromere-like region parS is replicated. Our understanding of how this sequence of events is regulated remains limited. The segregation of parS has been shown to involve multiple steps including polar release from anchoring protein PopZ, slow movement and fast ParA-dependent movement to the opposite cell pole. In this study, we demonstrate that ParA's competing attractions from PopZ and from DNA are critical for segregation of parS. Interfering with this balance of attractions - by expressing a variant ParA-R195E unable to bind DNA and thus favoring interactions exclusively between ParA-PopZ - results in cell death. Our data revealed that ParA-R195E's sole interactions with PopZ obstruct PopZ's ability to release the polar anchoring of parS, resulting in cells with multiple parS loci fixed at one cell pole. We show that the inability to separate and segregate multiple parS loci from the pole is specifically dependent on the interaction between ParA and PopZ. Collectively, our results reveal that the initial steps in chromosome segregation are highly regulated.