Protein phosphorylation on tyrosine has emerged as a key device in the control of numerous cellular functions in bacteria.In this article,we review the structure and function of bacterial tyrosine kinases and phosphatases.Phosphorylation is catalyzed by autophosphorylating adenosine triphosphate-dependent enzymes (bacterial tyrosine (BY) kinases) that are characterized by the presence of Walker motifs.The reverse reaction is catalyzed by three classes of enzymes:the eukaryotic-like phosphatases (PTPs) and dual-specific phosphatases; the low molecular weight protein-tyrosine phosphatases (LMW-PTPs); and the polymerase-histidinol phosphatases (PHP).Many BY kinases and tyrosine phosphatases can utilize host cell proteins as substrates,thereby contributing to bacterial pathogenicity.Bacterial tyrosine phosphorylation/dephosphorylation is also involved in biofilm formation and community development.The Porphyromonas gingivalis tyrosine phosphatase Ltp1 is involved in a restraint pathway that regulates heterotypic community development with Streptococcus gordonii.Ltp 1 is upregulated by contact with S.gordonii and Ltp 1 activity controls adhesin expression and levels of the interspecies signal Al-2.