Mutations in rpsL that confer streptomycin resistance show pleiotropic effects on virulence and the production of a carbapenem antibiotic in Erwinia carotovora

Spontaneous streptomycin-resistant derivatives of Erwinia carotovora subsp. carotovora strain ATTn10 were isolated. Sequencing of the rpsL locus (encoding the ribosomal protein S12) showed that each mutant was missense, with a single base change, resulting in the substitution of the wild-type lysine by arginine, threonine or asparagine at codon 43. Phenotypic analyses showed that the rpsL mutants could be segregated into two groups: K43R mutants showed reduced production of the beta-lactam secondary metabolite 1-carbapen-2-em-3 carboxylic acid (Car), but little effect on exoenzyme production or virulence in potato tuber tests. By contrast, the K43N and K43T mutations were pleiotropic, resulting in reduced exoenzyme production and virulence, as well as diminished Car production. The effect on Car production was due to reduced transcription of the quorum-sensing-dependent car biosynthetic genes. The effects of K43N and K43T mutations on Car production were partially alleviated by provision of an excess of the quorum-sensing signalling molecule N-(3-oxohexanoyl)-L-homoserine lactone. Finally, a proteomic analysis of the K43T mutant indicated that the abundance of a subset of intracellular proteins was affected by this rpsL mutation.