The Streptomyces coelicolor genome encodes a type I ribosome-inactivating protein

Abstract

Ribosome-inactivating proteins (RIPs) are cytotoxicN-glycosidases identified in numerous plants, but also constitute a subunit of the bacterial Shiga toxin. Classification of plant RIPs is based on the absence (type I) or presence (type II) of an additional lectin module. In Shiga toxin, sugar binding is mediated by a distinct RIP-associated homopentamer. In the genome of two actinomycetes, we identified RIP-like proteins that resemble plant type I RIPs rather than the RIP subunit (StxA) of Shiga toxin. Some representatives ofβ- andγ-proteobacteria also contain genes encoding RIP-like proteins, but these are homologous to StxA. Here, we describe the isolation and initial characterization of the RIP-like gene product SCO7092 (RIPsc) from the Gram-positive soil bacteriumStreptomyces coelicolor. Theripscgene was expressed inEscherichia colias a recombinant protein of about 30 kDa, and displayed the characteristicN-glycosidase activity causing specific rRNA depurination. InStreptomyces lividansandE. coli, RIPsc overproduction resulted in a dramatic decrease in the growth rate. In addition, intracellular production was deleterious forSaccharomyces cerevisiae. However, when applied externally to microbial cells, purified RIPsc did not display antibacterial or antifungal activity, suggesting that it cannot enter these cells. In a cell-free system, however, purifiedS. coelicolorRIPsc protein displayed strong inhibitory activity towards protein translation.