A Mycobacterial smc Null Mutant Is Proficient in DNA Repair and Long-Term Survival

SMC (structural maintenance of chromosomes) proteins play fundamental roles in various aspects of chromosome organization and dynamics, including repair of DNA damage. Mutant strains of Mycobacterium smegmatis and Mycobacterium tuberculosis defective in SMC were constructed. Surprisingly, inactivation of smc did not result in recognizable phenotypes in hallmark assays characteristic for the function of these genes. This is in contrast to data for smc null mutants in other species. Most bacterial cells contain a single circular chromosome that is folded and compacted into a structure called the nucle-oid (24). Diverse DNA binding proteins are associated with the nucleoid and take part in chromosome organization to assist in a variety of complex processes such as replication, recombina-tion, repair, modification, and transcription (8). Among these proteins are the SMC (structural maintenance of chromo-somes) proteins that are conserved from prokaryotes to eu-karyotes (4, 5, 28). The SMC family proteins are large proteins in the range between 110 and 170 kDa and share commo