Characterisation of the putative Type VII protein secretion system of Streptomyces spp
Streptomyces are Gram-positive bacteria belonging to the Actinobacteria. They are predominantly soil-dwelling saprophytic organisms. S. coelicolor is often used as a model organism for the study of the mechanisms underlying the processes of development and antibiotic production. S. scabies is one of only a few members of the genus that is pathogenic, being the causal agent of common scab of potato. Bioinformatic analysis of the genomes of both S. coelicolor and S. scabies revealed the presence of genes encoding components of putative Type VII protein secretion systems (T7SSs). To investigate the role of these putative T7SSs in S. coelicolor and S. scabies, marked deletions were constructed in genes encoding core components of the machinery. Phenotypic analysis revealed that deletion of genes encoding the T7SS FtsK/SpoIIIE ATPases (FSD) resulted in delayed actinorhodin production in S. coelicolor and slightly accelerated aerial development in S. scabies. In S. scabies, deletion of the genes encoding the WXG100 proteins, canonical substrates of T7SSs, led to a marked reduction in aerial hyphae formation and sporulation, in a media-dependent manner when compared to the wild-type strain. Closer inspection by scanning electron microscopy revealed that the spore chains of these mutant strains were under-developed and spores were irregularly sized. Strains of S. scabies lacking the WXG proteins (but not the FSD protein) were also resistant to lytic infection by the bacteriophage FC31. Taken together, these observations indicate that the major role of the WXG100 proteins in Streptomyces is intracellular.Plant infection assays indicate that the T7SS of S. scabies does not significantly contribute to virulence in several laboratory models of plant infection.
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