Roles of PknB and CslA in Cell Wall Morphogenesis of Streptomyces.

The bacterial cell wall is essential for maintaining cellular integrity and defining the mode of growth, with different species adopting distinct strategies for cell wall synthesis and remodelling. Streptomyces are filamentous bacteria predominantly found in soil and renowned for producing specialised metabolites, including antibiotics. They grow through tip extension and branching hyphal filaments, forming a multicellular mycelium. New branches are established by forming a new growth zone on the lateral cell wall. Proteins involved in this process are organised into complexes called polarisomes, with DivIVA being the most well-characterised component. To investigate the tip growth requirements in Streptomyces albus we developed a genetic screen utilising toxic DivIVA overproduction and searched for suppressors of its lethality, reasoning that such suppressors would likely encode components functionally linked to DivIVA or the tip growth machinery. Among the identified genes was pknB, encoding a serine/threonine protein kinase implicated in the regulation of cell growth and morphogenesis. We confirmed that deletion of pknB restored the growth phenotype of S. albus following DivIVA overproduction. The phosphoproteome analysis revealed that the absence of PknB alters the phosphorylation state of CslA, a cellulose synthase that synthesizes a beta-glucan (cellulose) polymer. We demonstrate that a phosphoablative mutant of CslA impairs beta-glucan synthesis and causes hypersensitivity to lysozyme. Overproduction of CslA restored colony growth defects arising from DivIVA-induced hyperbranching, without however suppressing the hyperbranching phenotype. These findings collectively identify PknB-dependent phosphorylation of CslA as a central regulatory point in Streptomyces cell envelope construction, revealing how modulation of beta-glucan synthesis can mitigate the cellular consequences of DivIVA dysregulation.