Genetic interaction of the SMC complex with topoisomerase IV in Bacillus subtilis

The role of topoisomerase IV (Topo IV) and of the structural maintenance of chromosomes (SMC) complex in chromosome compaction and in global protein synthesis was investigated. Lowering of the levels of Topo IV led to chromosome decondensation, while overproduction induced chromosome hyper-compaction, showing that Topo IV has an influence on the compaction of the whole chromosome, in a manner similar to that of the SMC protein, though different in mechanism. Increased synthesis of Topo IV in smc-deleted cells partially rescued the growth and condensation defect of the deletion, but not the segregation defect, revealing that the two systems interact at a genetic level. Two-dimensional gel investigations showed that global protein synthesis is highly aberrant in smc-deleted cells, and, to a different extent, also in cells lacking ScpA or ScpB, which form the SMC complex together with SMC protein. Overproduction of Topo IV partially rescued the defect in protein synthesis in smc mutant cells, indicating that Topo IV can restore the loss of negative supercoiling caused by the absence of SMC protein, but does not fully rescue the segregation defect. The data also show that the SMC protein has a dual function, in chromosome supercoiling and in active segregation.