A Laterally Acquired Galactose Oxidase-Like Gene Is Required for Aerial Development during Osmotic Stress in <em>Streptomyces coelicolor</em>

<div><p>Phylogenetic reconstruction revealed that most Actinobacterial orthologs of <em>S. coelicolor SCO2837</em>, encoding a metal-dependent galactose oxidase-like protein, are found within <em>Streptomyces</em> and were probably acquired by horizontal gene transfer from fungi. Disruption of <em>SCO2837</em> (<em>glxA)</em> caused a conditional <em>bld</em> phenotype that could not be reversed by extracellular complementation. Studies aimed at characterising the regulation of expression of <em>glxA</em> showed that it is not a target for other <em>bld</em> genes. We provide evidence that <em>glxA</em> is required for osmotic adaptation, although independently from the known osmotic stress response element SigB. <em>glxA</em> has been predicted to be part of an operon with the transcription unit comprising the upstream <em>cslA</em> gene and <em>glxA</em>. However, both phenotypic and expression studies indicate that it is also expressed from an independent promoter region internal to <em>cslA</em>. GlxA displays an <em>in situ</em> localisation pattern similar to that one observed for CslA at hyphal tips, but localisation of the former is independent of the latter. The functional role of GlxA in relation to CslA is discussed.</p>