Post-translational control of Bacillus subtilis biofilm formation mediated by tyrosine phosphorylation

A biofilm is a complex community of cells enveloped in a self-produced polymeric matrix. Entry into a biofilm is exquisitely controlled at the level of transcription and in the Gram-positive organism Bacillus subtilis it requires the concerted efforts of three major transcription factors. Here, we demonstrate that in addition to transcriptional control, B. subtilis utilizes post-translational modifications to control biofilm formation; specifically through phosphorylation of tyrosine residues. Through our work we have assigned novel roles during biofilm formation to two proteins; the protein tyrosine kinase PtkA and the protein tyrosine phosphatase PtpZ. Furthermore by introducing amino acid point mutations within the catalytic domains of PtkA and PtpZ we have identified that the kinase and phosphatase activities, respectively, are essential for function. PtkA contains a conserved C-terminal tyrosine cluster that is the site of autophosphorylation; however, our in vivo analysis demonstrates that this domain is not required during biofilm formation. With the aim of identifying the target(s) of PtkA controlled during biofilm formation we used a systematic mutagenesis approach but, despite extensive efforts, it remained elusive. Our findings highlight the complexity of biofilm development by revealing an additional level of regulation in the form of protein tyrosine phosphorylation.