These aerial structures are decorated with a hydrophobic coating of rodlets consisting of chaplins and rodlins. Here, we show that rodlins and the surface-active peptide SapB are essential for development during growth in a medium with high osmolarity. To this end, both vegetative and aerial hyphae secrete SapB, whereas rodlins are only secreted by the spore-forming aerial hyphae. Streptomycetes are filamentous bacteria with a complex life cycle. Spore germination and subsequent growth results in the formation of a substrate mycelium, which consists of a network of interconnected hyphae. Following a period of vegetative growth, aerial hyphae are formed that eventually septate into

chains of spores (Claessen et al., 2006). The chaplins (Claessen et al., 2003; Elliot et al., 2003) MLN0128 molecular weight and SapB (Willey et al., 1991; Tillotson et al., 1998;

Kodani et al., 2004; Capstick et al., 2007) Napabucasin in vitro have been shown to fulfill a role in spore formation. Two of eight of the chaplins, ChpE and ChpH, are secreted into the environment before aerial growth has started (Claessen et al., 2003). They lower the surface tension of the medium thereby enabling hyphae to grow into the air (Claessen et al., 2003; Sawyer et al., 2011). Aerial hyphae secrete all chaplins, ChpA-H, which assemble on the hyphal surface into an amphipathic protein film that consists of amyloid-like fibrils (Claessen et al., 2003, 2004; Capstick et al., 2011; Sawyer et al., 2011). The rodlin proteins organize these chaplin fibrils into so-called rodlets (Claessen et al., 2004). Yet, under the conditions tested, rodlins were not essential for development (Claessen et al., 2002). SapB is a lantibiotic-like peptide of 2027 Da (Willey et al., 1991; Kodani et al., 2004). Like ChpE and ChpH, SapB lowers the surface tension and thus allows hyphae to grow

into the air (Tillotson et al., 1998; Capstick et al., 2007). Production of SapB is encoded and controlled by the ramCSABR gene cluster. SapB is derived from the 42 amino acid prepeptide encoded by ramS, which is probably post-translationally modified by the action of RamC (O’Connor et al., 2002; Kodani et al., 2004; Willey et al., 2006). The ABC-transporter encoded by 3-mercaptopyruvate sulfurtransferase ramAB is generally believed to transport SapB outside of the cell (Kodani et al., 2004; Willey et al., 2006), while RamR is the transcriptional regulator that controls expression of ramCSAB (Keijser et al., 2002; O’Connor et al., 2002). Interestingly, SapB was shown to be required for differentiation on certain complex media, but not on minimal media with mannitol as the carbon source (Willey et al., 1991). Here, we show that this difference is because of the osmolarity of the medium. We furthermore demonstrate that in addition to SapB, the rodlet layer contributes to efficient aerial growth when hyphae encounter osmotic stress conditions.