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Tyrosine kinases in prokaryotic organisms and SSB proteins

Over decades general assumption was that tyrosine phosphorylation is restricted to eukaryotic cells. However, over the last decades this modification on tyrosine has been discovered in many bacteria. Recently it has been reported that single stranded DNA binding protein (SSB) is phosphorylated on tyrosine. SSB protein binds and stabilizes single-stranded DNA (ssDNA) during bacterial growth. It has also been reported that phosphoralytion of SSB modulates its affinity for DNA. This specific modification has been found in distantly related bacteria such as S. coelioclor, B. subtilis and E. coli. E. coli has two distinct tyrosine kinases, Wzc and Etk. To examine which kinase phosphorylates SSB in vivo, etk and wzc genes of the E.coli NM522 chromosome were disrupted. Here we report the results on tyrosine phosphorylation of SSB protein isolated from wild type and kinase-defficiant mutants. In parallel, we continue investigation on tyrosine phosphorylation of SSB protein from S. coelicolor. The tyrosine kinase(s) in this bacterium is still not identified. SSB phosphorylation site was predicted by 3D modeling and comparison with B. subtilis SSB. In order to confirm this prediction, the point mutation of Tyr88 on SSB was introduced by PCR-overlap mutagenesis. Interestingly, our result shows that extent of phosphorylation of mutant and wild type SSB is the same. This suggests that a) the predicted site is not correct, or b) more than one tyrosine is phosphorylated in SSB protein.

tyrosine kinases; SSB proteins

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