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The Rhizobium meliloti ExoK and ExsH glycanases specifically depolymerize nascent succinoglycan chains
1Department of Biology, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
Summary
Rhizobium meliloti ExoK and ExsH glycanases depolymerize high molecular weight succinoglycan. These enzymes cleave newly synthesized succinoglycan, regulating its molecular weight distribution.
Area of Science:
- Microbiology
- Bacteriology
- Glycobiology
Background:
- Succinylglycan is a key exopolysaccharide produced by Rhizobium meliloti.
- The molecular weight of succinoglycan influences its function in bacterial signaling and symbiosis.
- ExoK and ExsH glycanases were previously hypothesized to reduce succinoglycan chain length.
Purpose of the Study:
- To investigate the enzymatic activity of Rhizobium meliloti ExoK and ExsH glycanases on succinoglycan.
- To elucidate the role of ExoK and ExsH in the production of low molecular weight (LMW) succinoglycan.
Main Methods:
- Expression and purification of ExoK and ExsH proteins.
- Enzymatic assays using succinoglycan treated with various conditions (heat, acid).
- Western blot analysis to detect protein expression and extracellular localization.
- Comparison of purified enzyme activity with in-culture depolymerization.
Main Results:
- Purified ExoK and ExsH showed limited cleavage of accumulated succinoglycan.
- Pre-treatment of succinoglycan (heat, acid) enhanced its susceptibility to cleavage by ExoK and ExsH.
- ExoK and ExsH are expressed and secreted extracellularly by R. meliloti.
- Enzymes efficiently cleave actively synthesized, nascent succinoglycan, but not aged succinoglycan.
Conclusions:
- ExoK and ExsH dynamically regulate succinoglycan molecular weight by cleaving nascent chains.
- Enzymatic activity is restricted to a specific post-synthesis window, likely before conformational changes.
- This regulation ensures the production of functional LMW succinoglycan in R. meliloti cultures.