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Tyrosine phosphorylation of bacterial SSBs from taxonomically distant bacterial species

Single stranded DNA binding proteins (SSBs) are essential proteins required in various stages of DNA metabolism. SSBs bind DNA in a sequence independent manner and maintain genome integrity during DNA replication, recombination or repair. The main role of SSB protein in the cell is to stabilize single-stranded DNA (ssDNA). It is also known that SSBs interact and modulate the activities of many enzymes such as DNA polymerase, RNA polymerase or DNA helicase. Bacterial SSBs are homo-tetramers, while eukaryotic are hetero-trimers. Although they differ considerably in the structure, these enzymes accomplish similar functions. Interestingly, eukaryotic SSBs are regulated by phosphorylation on several serine and threonine residues, while our recent discoveries pointed out that bacterial SSBs are phosphorylated on tyrosine residue(s). The first evidence has been obtained by immunoaffinity chromatography. We have purified tyrosine phosphorylated SSB from Streptomyces griseus. Since genes encoding protein-tyrosine kinases have not been recognized in streptomycetes and SSBs from Streptomyces coelicolor and Bacillus subtilis share 40% identity, we used B. subtilis protein-tyrosine kinase YwqD to examine phosphorylation of the two cognate SSBs (SSB and YwpH) in vitro. Focusing on this model system we found that phosphorylation of SSB is affected antagonistically by B. subtilis tyrosine kinase (YwqD) and phosphatase (YwqE). We showed next that phosphorylation of B. subtilis SSB significantly increased binding to single-stranded DNA in vitro. We also found decrease in SSB phosphorylation under DNA-damaging conditions in the cells. A significantly lower extent of phosphorylation suggested one biological aspect of this modification in the regulation

Tyrosine phosphorylation ; single-stranded DNA binding proteins ; SSB

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