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Translational attenuation of ermC: a deletion analysis
Abstract:
ermC is a plasmid gene which specifies resistance to macrolide-lincosamide-streptogramin B antibiotics. The product of ermC was previously shown to be an inducible rRNA methylase, which is regulated translationally, and a mechanism for this regulation, termed the translational attenuation model, has been proposed. This model postulates that alternative inactive and active conformational states of the ermC mRNA are modulated by erythromycin-induced ribosome-stalling during translation of a leader peptide. In the present study the translational attenuation model was tested by constructing a series of deletants missing the ermC promoter and portions of the regulatory (leading) region. In these mutants, ermC transcription is dependent on fusion to an upstream promoter. Depending on the terminus of each deletion within the regulatory region, determined by DNA sequencing, ermC expression is observed to be either high level and inducible (like the wild-type), high level and noninducible, or low level and noninducible. The translational attenuation model predicts that as the deletions extend deeper into the leader region, successively masking and unmasking sequences required for translation of the methylase, an alternation of high and low level methylase expression will be observed. These predictions are confirmed. Based on this and other information, the model is refined and extended, and both direct translational activation and kinetic trapping of a metastable active intermediate during transcription are proposed to explain basal synthesis of methylase and to rationalize the effects of certain regulatory mutants.
Insights
The ermC gene
Area of Science:
- Molecular Biology
- Microbiology
- Genetics
Background:
- The ermC gene confers resistance to macrolide-lincosamide-streptogramin B antibiotics.
- Its product, an rRNA methylase, is translationally regulated via a proposed translational attenuation model.
- This model involves erythromycin-induced ribosome stalling affecting ermC mRNA conformation.
Purpose of the Study:
- To experimentally test the proposed translational attenuation model for ermC gene regulation.
- To investigate the role of specific regulatory regions within the ermC leader sequence.
- To refine and extend the understanding of ermC gene expression control.
Main Methods:
- Construction and analysis of a series of ermC deletion mutants.
- Fusion of ermC to an upstream promoter to control transcription.
- DNA sequencing to determine deletion endpoints within the regulatory region.
Main Results:
- Deletion mutants exhibited varied ermC expression: high-level inducible, high-level noninducible, and low-level noninducible.
- Observed expression patterns confirmed predictions of the translational attenuation model regarding leader region sequences.
- Specific deletions correlated with altered translational control and methylase expression levels.
Conclusions:
- The translational attenuation model is supported by experimental evidence from deletion mutants.
- Refined model incorporates direct translational activation and kinetic trapping for basal methylase synthesis.
- Understanding ermC regulation provides insights into antibiotic resistance mechanisms.