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Distinct properties of two paralogous SSB proteins from Streptomyces coelicolor

SSB proteins that have been identified in all domains of life and in viruses are essential for cell survival. These proteins bind and protect single stranded DNA. However, during DNA replication, recombination and repair, the cellular role of SSBs is not passive. SSB proteins interact and modulate the functions of other proteins involved in all aspects of DNA metabolism. Comprehensive analysis of SSB interactions gives emphasis to its prominent role in the complex protein network responsible for genome maintenance. Our recent bioinformatics study showed that many bacterial genomes possess more than one ssb gene. The cellular role of additional SSB is not well understood. Therefore, Streptomyces coelicolor, bacterium with complex life style that also possesses two SSBs was chosen as a good model system to study biological role(s) of paralogous SSB proteins. We demonstrated that two ssb genes in S. coelicolor are differentially regulated. Expression of ssbA is constant and high during life cycle, decreasing towards late stationary phase, while the expression of ssbB is at all-time low. In minimal medium the ssbB is significantly up regulated. Promoters for both genes are determined. Surprisingly, promoter of ssbA possesses unusual features and is active in taxonomically distant bacteria, Escherichia coli. Both genes were disrupted on the genome and our results showed that SSB-A is essential for survival while SSB-B is important during sporulation process. In addition, the 3D structure of SSB-B was solved, showing an interesting variation but also high resemblance to the quaternary structure of the previously published S. coelicolor SSB-A protein. This is the first example of solved crystal structures of two paralogous SSB proteins from the same organism.

DNA metabolism ; SSB protein ; sporulation ; Streptomyces

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