Characterisation of ParB encoded on multipartite genome in Deinococcus radiodurans and their roles in radioresistance.
The Deinococcus radiodurans multipartite genome consists of 2 chromosomes and 2 plasmids Its genome encodes 4 ParA and 4 ParB proteins on different replicons. Multiple sequence alignments of ParBs encoded on these genome elements showed that ParB of primary chromosome (ParB1) is close to chromosomal type ParB and is found to be different from ParBs encoded on chromosome II (ParB2) and megaplasmid (ParB3) elements. We observed that ParB1, ParB2 and ParB3 exist as dimer in solution and these proteins interact to self but not to its homologs in D. radiodurans, suggesting the specificity in ParBs dimerization. The parB1 deletion mutant showed slow growth under normal condition and relatively reduced resistance to γ-radiation as compared to wild type. The parB2 and parB3 mutants maintained without selection pressure showed loss of radioresistance, which was not observed when maintained with selection pressure. Nearly half of the populations of these mutants showed resistance to antibiotics marked to respective genome elements. Interestingly, all the parB mutants showed increased copy numbers of cognate genome element in cells maintained with antibiotics possibly due to arrest in genome segregation. These results suggested that ParB proteins encoded on multipartite genome system in D. radiodurans form homodimer and not heterodimer with other ParB homologs, and they independently regulate the segregation of respective genome elements. The roles of ParB1 proteins in normal as well as radiation stressed growth of this bacterium have also been ascertained.