engleski

What will they do if there are two?

SSB proteins are essential for survival, from bacteria to human. They bind ssDNA with a high affinity and in a sequence independent manner thus protecting ssDNA transiently formed in DNA metabolic processes. SSBs interact and modulate the activities of various proteins involved in DNA metabolism. Our recent analysis of available bacterial genomes revealed the presence of multiple copies of genes encoding SSB proteins in many bacteria. Moreover, we have noticed that the number of ssb genes can vary even among closely related species thus indicating that evolution of these proteins in Eubacteria is highly dynamic. However, the role of duplicated SSB proteins is poorly studied. Therefore, we have started this research asking: What they will do if they are two? Multicellular bacterium Streptomyces coelicolor was selected as a model system since it has two ssb genes. Gene expression analyses suggested that SsbA and SsbB may have different biological roles. In concert with this, the EMSA assays and fluorescent titrations showed that SsbA and SsbB bind to ssDNA with different affinity. Crystal structures of these proteins also revealed some structural variations that led us to hypothesize that SsbB binding activity might be regulated during oxidative stress in S. coelicolor. In addition, results of the gene disruptions have strongly indicated that ssbA is essential for survival while ssbB is important during the sporulation process. To get a better insight into the function of SsbB we examined the impact of disulfide bridges removal on SsbB activity/stability. The gene ssbB carrying cys7 mutation was not able to complement S. coelicolor strain lacking ssbB. To shed the light on the complex mechanism of cell division in Streptomyces, we have constructed double (ssbBparB ; ssbBsmc) and triple (ssbBparBsmc) mutant strains carrying mutations in ssbB gene and in the genes previously reported to be important for the chromosome segregation. The results showed more severe defects in nucleoid segregations during sporulation than previously reported for parental strains. Finally, by fluorescence microscopy we examined the localization of SsbA and SsbB proteins in streptomycete hyphae and observed occasional co-localization for these two paralogous proteins.

SSB proteins; Streptomyces coelicolor

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