Consequently, unnecessary expression of secondary metabolites can be costly to organisms producing secondary metabolites. On the other hand, secondary metabolism poses metabolic burdens to the producer by diverting precursors, cofactors, and energy from primary metabolism ( 7). For example, a secondary metabolite with antimicrobial activity may allow organisms to inhibit competitors that occupy the same niches ( 6). Secondary metabolites are thought to represent important adaptive characters of the organisms producing secondary metabolites in their native environments ( 2, 5). Diverse classes of secondary metabolites are produced by plants, animals, and microorganisms, but knowledge of the natural functions of secondary metabolites is still limited. These metabolites vary widely in structure and biological activity, with some having valuable pharmaceutical and agricultural applications ( 3, 4). Secondary metabolites play important roles in physiological adaptation and ecologic fitness of the organisms producing secondary metabolites ( 1, 2). 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.
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. Pyoluteorin biosynthesis, which poses a metabolic burden on the producer cells, but not pyoluteorin itself, leads to the accumulation of the spontaneous mutants. Our results showed that secondary metabolism, specifically biosynthesis of the antimicrobial compound pyoluteorin, contributes significantly to the accumulation of Gac − mutants.
Here we investigated the role of secondary metabolism in the accumulation of Gac − mutants in Pseudomonas protegens strain Pf-5. Intriguingly, spontaneous mutations in gacS or gacA (Gac − mutants) are commonly observed in laboratory cultures. In Pseudomonas spp., secondary metabolism is controlled by the GacS-GacA global regulatory system. Secondary metabolism also can be costly, as it shunts energy and intermediates from primary metabolism. Secondary metabolites are synthesized by many microorganisms and provide a fitness benefit in the presence of competitors and predators. PEX18Km with a 1,160-bp synthesized DNA fragment, containing pltR of Pf-5 with The promoter of pltL fused with a promoterless gfp PPROBE′-gfp (tagless) contains the intergenic region between pltR and pltL, including
PEX18Tc containing wild-type pltA in a 1,359-bp BamHI fragment Gene replacement vector with MCS from pUC18 sacB + Tc r Secondary metabolism and many other phenotypes regulated by GacA Δ gacA mutant contains a 612-bp deletion in gacA (PFL_3563) of Pf-5 altered in the PltR* in the chromosome of Pf-5 overexpresses plt biosynthesis genes Plt − PltR* Δ pltA double mutant contains a 275-bp deletion in pltA and the codon-modified Overexpresses plt biosynthesis genes and overproduces pyoluteorin PltR* mutant contains codon-modified pltR (PFL_2785) in the chromosome of Pf-5 Δ rzxB mutant contains a 1,342-bp deletion in rzxB (PFL_2989) of Pf-5 Rzx − Δ prnC mutant contains an 86-bp insertion of FRT site in prnC (PFL_3606) of Pf-5 Prn − PltA+ complemented mutant contains a wild-type pltA gene replacing the mutated pltA Δ pltA mutant contains a 275-bp deletion in pltA (PFL_2787) of Pf-5 Plt − Δ phlA mutant contains a 639-bp deletion in phlA (PFL_5954) of Pf-5 MAPG − DAPG − Δ ofaA mutant contains 1,143-bp deletion in ofaA (PFL_2145) of Pf-5 contains FRT scar Δ hcnB mutant contains a 239-bp deletion in hcnB of Pf-5 HCN − Description (genotype and/or relevant characteristics a)Ħ-fold mutant contains mutations in hcnB, ofaA, phlA, pltA, prnC, and rzxB of strain Pf-5ĭescribed below HCN − Ofa − DAPG − MAPG − Plt − Prn − Rzx −
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