The MreB-like protein Mbl of S. coelicolor A3(2) depends on MreB for proper localization and contributes to spore wall synthesis.

Most bacteria with a rod-shape morphology contain an actin-like cytoskeleton consisting of MreB polymers which form helical spirals underneath the cytoplasmic membrane to direct peptidoglycan synthesis for elongation of the cell wall. In contrast MreB of Streptomyces coelicolor is not required for vegetative growth, but has a role in sporulation. Beside MreB, S. coelicolor encodes two further MreB-like proteins, Mbl and SCO6166, whose function is unknown. Whereas MreB and Mbl are highly similar, SCO6166 is shorter, lacking the subdomains IB and IIB of actin-like proteins. Here we showed that MreB and Mbl are not functionally redundant but cooperate in spore wall synthesis. Expression analysis by semi-quantitative RT-PCR revealed distinct expression patterns. mreB and mbl are predominantly induced during morphological differentiation. In contrast sco6166 is strongly expressed during vegetative growth but switched off during sporulation. All genes could be deleted without affecting viability. Even a ΔmreB/mbl double mutant was viable. Δsco6166 had a wildtype phenotype. ΔmreB, Δmbl and ΔmreB/mbl produced swollen prematurely germinating spores that were sensitive to various kinds of stress, suggesting a defect in spore wall integrity. During aerial mycelium formation an Mbl-mCherry fusion protein colocalized with an MreB-eGFP fusion protein at the sporulation septa. Whereas MreB-eGFP localized properly in the Δmbl mutant, Mbl-mCherry localization depended on the presence of a functional MreB protein. Our results revealed that MreB and Mbl cooperate in synthesis of the thickened spore wall, while SCO6166 has a non-essential function during vegetative growth.