Genetic analysis of the chromosome segregation protein SPoOj of Bacillus subtilis: Evidence for separate domains involved in DNA binding and interactions with SOj protein

Abstract

SpoOJ and Soj belong to the ParB/ParA family of proteins involved in chromosome and plasmid segregation in bacteria. In Bacillus subtilis, SpoOJ protein binds to several specific sites, parS, located on both sides of the origin of DNA replication, oriC, and apparently self-associates to form large discrete foci visible by fluorescence microscopy. Soj protein forms large 'patches' probably associated with the nucleoid, which can undergo dynamic, co-operative jumping from nucleoid to nucleoid in the presence of SpoOJ. Patches of Soj protein somehow help to bring about the condensation of SpoOJ foci. Soj is also a negative regulator of transcription. In the absence of SpoOJ, Soj is statically distributed on each of the nucleoids in the cell and blocks the transcription of several sporulation genes. To analyse the functional interaction between SpoOJ and Soj further, we have constructed and studied a collection of spoOJ mutants. Most of the mutants completely prevent SpoOJ from interacting with DNA. One mutation impairs the formation of compact SpooJ foci and simultaneously results in loss of Soj movement. We also isolated one spoOJ mutant, in which the frequency of Soj internucleoid oscillation is highly increased. Both mutations affecting the interaction with Soj lie in the N-terminal coding part of spoOJ, whereas the substitutions affecting DNA binding lie in the mid- to C-terminal coding region.