Structure–function relationships of two paralogous single-stranded DNA-binding proteins from Streptomyces coelicolor: implication of SsbB in chromosome segregation during sporulation

Tina Paradzik, Nives Ivic, Zelimira Filic, Babu Manjasetty, Paul Herron, Marija Luic, Dusica Vujaklija

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The linear chromosome of Streptomyces coelicolor contains two paralogous ssb genes, ssbA and ssbB. Following mutational analysis, we concluded that ssbA is essential, whereas ssbB plays a key role in chromosome segregation during sporulation. In the ssbB mutant, 0% of spores lacked DNA. The two ssb genes were expressed differently; in minimal medium, gene expression was prolonged for both genes and significantly upregulated for ssbB. The ssbA gene is transcribed as part of a polycistronic mRNA from two initiation sites, 163 bp and 75 bp upstream of the rpsF translational start codon. The sbB gene is transcribed as a monocistronic mRNA, from an unusual promoter region, 73 bp upstream of the AUG codon. Distinctive DNA-binding affinities of single-stranded DNA-binding proteins monitored by tryptophan fluorescent quenching and electrophoretic mobility shift were observed. The crystal structure of SsbB at 1.7A ° resolution revealed a common OB-fold, lack of the clamp-like structure conserved in SsbA and previously unpublished S-S bridges between the A/B and C/D subunits. This is the first report of the determination of paralogous singlestranded DNA-binding protein structures from the same organism. Phylogenetic analysis revealed frequent duplication of ssb genes in Actinobacteria, whereas their strong retention suggests that they are involved in important cellular functions.
Original languageEnglish
Pages (from-to)3659-3672
Number of pages14
JournalNucleic Acids Research
Issue number6
Early online date7 Feb 2013
Publication statusPublished - Apr 2013


  • microbiology
  • chromosome segregation
  • cell biology
  • structure–function relationships
  • paralogous single-stranded DNA-binding proteins
  • streptomyces coelicolor
  • SsbB
  • sporulation
  • chromosome segregation

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