Recombinant Rhizobium meliloti Exopolysaccharide II synthesis transcriptional activator expG (expG)
Description
Regulatory Mechanisms
ExpG activates EPS II biosynthesis under phosphate-limiting conditions and in the presence of extra expG copies . Its activity is modulated by:
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Phosphate Regulation: PhoB binds PHO box-like sequences in exp promoters, synergizing with ExpG under phosphate starvation .
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MucR Mutation: Deletion of mucR (a repressor) elevates exp transcription, further enhanced by phosphate limitation .
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Self-Induction: Extra expG copies upregulate all exp complementation groups .
Table 2: Regulatory Interactions Governing ExpG Activity
DNA Binding and Promoter Interactions
ExpG binds to conserved promoter regions upstream of expA1, expG/expD1, and expE1 . Key findings include:
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Binding Site: A 28-bp palindromic sequence with two flanking motifs .
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Binding Kinetics: Atomic force microscopy (AFM) revealed unbinding forces of 50–165 pN and dissociation rates of ~1.2 × 10⁻³ s⁻¹ .
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Promoter Regulation: ExpG activates proximal promoters of expA, expD, and expE operons, while MucR represses distal promoters .
Table 3: ExpG Binding Site Characteristics
Functional Role in Symbiosis and Stress Adaptation
EPS II production is critical for root colonization and symbiosis with legumes like alfalfa . ExpG’s role includes:
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Symbiotic Efficiency: EPS II promotes nodule formation and nitrogen fixation .
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Stress Resistance: EPS II protects against osmotic and oxidative stress .
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Regulatory Cross-Talk: ExpG and PhoB integrate phosphate signaling with symbiotic requirements .
Table 4: Functional Outcomes of ExpG Activity
Research Implications and Future Directions
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Recombinant ExpG: While not explicitly studied, recombinant ExpG could enable heterologous EPS II production in industrial microbes or biofilm engineering .
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Therapeutic Potential: ExpG’s regulatory mechanisms may inspire novel biotechnological strategies for controlling polysaccharide biosynthesis .
Product Specs
Target Background
KEGG: sme:SM_b21317
