SMC modulates ParB engagement in segregation complexes in Streptomyces

Abstract

ParB is long established chromosome segregation protein in bacteria. Due to the recently demonstrated CTPase activity of ParB, formation of its nucleoprotein complexes was unrevealed. ParB homodimers bound to CTP are loaded onto DNA at parS sites, where they recruit condensin (SMC), thereby facilitating chromosome organization and segregation. Whether SMC modulates ParB complexes has remained unknown. Here, we generated Streptomyces venezuelae strains producing ParB-HaloTag in the presence or absence of SMC and used single-cell time-lapse fluorescence microscopy, single molecule tracking and fluorescence recovery after photobleaching analysis to explore ParB dynamics. Additionally, we performed chromatin immunoprecipitation to examine ParB interactions with DNA in the presence or absence of SMC. We reveal that SMC modulates ParB complex stability on DNA. We find that the absence of SMC results in faster ParB complex disassembly, and promotes non-specific DNA binding. Additionally, we show that SMC reduces ParB CTPase activity in vitro. Taken together our data provide evidence of SMC positive feedback on the ParB nucleoprotein complex, offering new insight into the nature of ParB complex regulation.