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Evidence for two phosphonate degradative pathways in Enterobacter aerogenes
K S Lee1, W W Metcalf, B L Wanner
1Department of Biological Sciences, Purdue University, West Lafayette, Indiana 47907.
Abstract:
We screened mini-Mu plasmid libraries from Enterobacter aerogenes IFO 12010 for plasmids that complement Escherichia coli phn mutants that cannot use phosphonates (Pn) as the sole source of phosphorus (P). We isolated two kinds of plasmids that, unexpectedly, encode genes for different metabolic pathways. One kind complements E. coli mutants with both Pn transport and Pn catalysis genes deleted; these plasmids allow degradation of the 2-carbon-substituted Pn alpha-aminoethylphosphonate but not of unsubstituted alkyl Pn. This substrate specificity is characteristic of a phosphonatase pathway, which is absent in E. coli. The other kind complements E. coli mutants with Pn catalysis genes deleted but not those with both transport and catalysis genes deleted; these plasmids allow degradation of both substituted and unsubstituted Pn. Such a broad substrate specificity is characteristic of a carbon-phosphorus (C-P) lyase pathway, which is common in gram-negative bacteria, including E. coli. Further proof that the two kinds of plasmids encode genes for different pathways was demonstrated by the lack of DNA homology between the plasmids. In particular, the phosphonatase clone from E. aerogenes failed to hybridize to the E. coli phnCDEFGHIJKLMNOP gene cluster for Pn uptake and degradation, while the E. aerogenes C-P lyase clone hybridized strongly to the E. coli phnGHIJKLM genes encoding C-P lyase but not to the E. coli phnCDE genes encoding Pn transport. Specific hybridization by the E. aerogenes C-P lyase plasmid to the E. coli phnF, phnN, phnO, and phnP genes was not determined. Furthermore, we showed that one or more genes encoding the apparent E. aerogenes phosphonatase pathway, like the E. coli phnC-to-phnP gene cluster, is under phosphate regulon control in E. coli. This highlights the importance of Pn in bacterial P assimilation in nature.
Insights
Researchers identified two distinct plasmid types from Enterobacter aerogenes that enable Escherichia coli to metabolize phosphonates (Pn), a phosphorus source. These plasmids encode either a phosphonatase or a C-P lyase pathway, crucial for bacterial phosphorus assimilation.
Area of Science:
- Microbiology
- Molecular Biology
- Biochemistry
Background:
- Phosphonates (Pn) are vital phosphorus sources for bacteria, but their assimilation pathways are not fully understood.
- Escherichia coli mutants lacking phosphonate utilization genes provide a model for studying these pathways.
- Enterobacter aerogenes harbors genetic elements that can complement these deficiencies.
Purpose of the Study:
- To identify and characterize plasmids from Enterobacter aerogenes that enable phosphonate utilization in Escherichia coli.
- To elucidate the specific metabolic pathways encoded by these plasmids.
- To investigate the genetic organization and regulation of these phosphonate assimilation pathways.
Main Methods:
- Screening of mini-Mu plasmid libraries from Enterobacter aerogenes in phosphonate-utilization deficient E. coli mutants.
- Characterization of plasmid-encoded phosphonate degradation capabilities and substrate specificities.
- DNA hybridization studies to assess homology with known E. coli phosphonate utilization gene clusters.
- Investigation of phosphate-dependent gene regulation.
Main Results:
- Two distinct plasmid types were isolated, each encoding a different phosphonate metabolic pathway.
- One plasmid type conferred a phosphonatase pathway, degrading specific substituted phosphonates.
- The other plasmid type conferred a broad-specificity C-P lyase pathway, degrading both substituted and unsubstituted phosphonates.
- These pathways showed no DNA homology and distinct substrate specificities, differentiating them from E. coli's native pathways.
- The phosphonatase pathway genes are under phosphate regulon control in E. coli.
Conclusions:
- Enterobacter aerogenes possesses distinct phosphonate assimilation pathways, including a phosphonatase pathway absent in E. coli.
- These findings reveal novel mechanisms for phosphonate utilization in bacteria.
- The identified pathways are important for bacterial phosphorus assimilation in natural environments.
- Understanding these pathways can inform strategies for managing phosphorus in microbial ecosystems.