Detecting posttranslational modifications of bacterial SSB proteins.

Posttranslational modifications of single-stranded DNA-binding proteins (SSBs), which are essential proteins in DNA metabolism, have been reported from prokaryotic to eukaryotic systems. While eukaryotic SSBs are regulated by phosphorylation on serine and threonine residues, bacterial SSB proteins are also phosphorylated on tyrosine residues. This was initially observed during a systematic search for global phosphotyrosine-containing proteins in Streptomyces, complex life cycle bacteria that support mycelial growth and spore formation. Tyrosine phosphorylation was further confirmed on SSB proteins from the spore-forming bacterium Bacillus subtilis and in the simpler prokaryote, Escherichia coli. However, a thorough study of this modification and its cognate kinase has been performed only on SSB proteins from Bacillus subtilis. It was shown that phosphorylation of B. subtilis SSB (SsbA) significantly increases binding affinity with single-stranded DNA in vitro. Mass spectrometry analysis of SsbA identified Tyr82 as the phosphorylation site. Analyses of the resolved and predicted crystal structures of SSB proteins from B. subtilis, E. coli, and S. coelicolor revealed that the Tyr phosphorylation site occupies similar positions in all three structures. Our results indicate that tyrosine phosphorylation of bacterial SSBs is a conserved modification in taxonomically distant bacteria.