Secondary Metabolism and Interspecific Competition Affect Accumulation of Spontaneous Mutants in the GacS-GacA Regulatory System in Pseudomonas protegens.

Qing Yan,Lucas D Lopes,Fernando D Andreote,Kerry L Mcphail,Joyce E Loper,Brenda T Shaffer,Chunxu Song,Jeff H Chang,Teresa A Kidarsa,Jos M Raaijmakers,Benjamin Philmus,Oliver Vining,Virginia O Stockwell

doi:10.1128/mbio.01845-17

Abstract

ABSTRACTSecondary metabolites are synthesized by many microorganisms and provide a fitness benefit in the presence of competitors and predators. Secondary metabolism also can be costly, as it shunts energy and intermediates from primary metabolism. In Pseudomonas spp., secondary metabolism is controlled by the GacS-GacA global regulatory system. Intriguingly, spontaneous mutations in gacS or gacA (Gac− mutants) are commonly observed in laboratory cultures. Here we investigated the role of secondary metabolism in the accumulation of Gac− mutants in Pseudomonas protegens strain Pf-5. Our results showed that secondary metabolism, specifically biosynthesis of the antimicrobial compound pyoluteorin, contributes significantly to the accumulation of Gac− mutants. Pyoluteorin biosynthesis, which poses a metabolic burden on the producer cells, but not pyoluteorin itself, leads to the accumulation of the spontaneous mutants. Interspecific competition also influenced the accumulation of the Gac− mutants: a reduced proportion of Gac− mutants accumulated when P. protegens Pf-5 was cocultured with Bacillus subtilis than in pure cultures of strain Pf-5. Overall, our study associated a fitness trade-off with secondary metabolism, with metabolic costs versus competitive benefits of production influencing the evolution of P. protegens, assessed by the accumulation of Gac− mutants.

Highlights

Secondary metabolites are synthesized by many microorganisms and provide a fitness benefit in the presence of competitors and predators
We compared the frequency with which spontaneous GacϪ mutants accumulate in four species of Pseudomonas, including Pseudomonas protegens Pf-5, Pseudomonas fluorescens SS101, Pseudomonas aeruginosa PAO1, and Pseudomonas chlororaphis 30-84
These strains were cultured in NYB, a medium in which P. protegens CHA0 was previously shown to accumulate GacϪ mutants [21]

Summary

Introduction

Secondary metabolites are synthesized by many microorganisms and provide a fitness benefit in the presence of competitors and predators. Our study associated a fitness trade-off with secondary metabolism, with metabolic costs versus competitive benefits of production influencing the evolution of P. protegens, assessed by the accumulation of GacϪ mutants. To balance the benefits and costs, secondary metabolism is controlled by complex regulatory networks that regulate production in response to environmental factors and physiological status of the organism producing secondary metabolites [8, 9]. These networks involve both global and pathway-specific regulators operating to coordinate secondary metabolite biosynthesis pathways at transcriptional and posttranscriptional levels [10,11,12]. Colonies of these mutants can be distinguished from those of the wild type visually by their expanded colony size, flattened appearance, increased fluorescence under UV light, and lack of extracellular protease production [21, 24]

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Results

Conclusion