Genetic Interactions of smc , ftsK , and parB Genes in Streptomyces coelicolor and Their Developmental Genome Segregation Phenotypes

The mechanisms by which chromosomes condense and segregate during developmentally regulated cell division are of interest forStreptomyces coelicolor , a sporulating, filamentous bacterium with a large, linear genome. These processes coordinately occur as many septa synchronously form in syncytial aerial hyphae such that prespore compartments accurately receive chromosome copies. Our genetic approach analyzed mutants forftsK ,smc , andparB . DNA motor protein FtsK/SpoIIIE coordinates chromosome segregation with septum closure in rod-shaped bacteria. SMC (s tructuralm aintenance ofc hromosomes) participates in condensation and organization of the nucleoid. ParB/Spo0J partitions the origin of replication using a nucleoprotein complex, assembled at a centromere-like sequence. Consistent with previous work, we show that anftsK -null mutant produces anucleate spores at the same frequency as the wild-type strain (0.8%). We report that thesmc andftsK deletion-insertion mutants (ftsK ′ truncation allele) have developmental segregation defects (7% and 15% anucleate spores, respectively). By use of these latter mutants, viable double and triple mutants were isolated in all combinations with a previously describedparB -null mutant (12% anucleate spores).parB andsmc were in separate segregation pathways; the loss of both exacerbates the segregation defect (24% anucleate spores). For a triple mutant, deletion of the region encoding the FtsK motor domain and one transmembrane segment partially alleviates the segregation defect of thesmc parB mutant (10% anucleate spores). Considerable redundancy must exist in this filamentous organism because segregation of some genomic material occurs 90% of the time during development in the absence of three functions with only a fourfold loss of spore viability. Furthermore, we report thatscpA andscpAB mutants (encoding SMC-associated proteins) have spore nucleoid organization defects. Finally, FtsK-enhanced green fluorescent protein (EGFP) localized as bands or foci between incipient nucleoids, while SMC-EGFP foci were not uniformly positioned along aerial hyphae, nor were they associated with every condensing nucleoid.