Unraveling the Role of Silent Mutation in the ω-Subunit of Escherichia coli RNA Polymerase: Structure Transition Inhibits Transcription.

The bacterial RNA polymerase is a multi-subunit enzyme complex composed of six subunits, α2ßß'σω. The function of this enzyme is to transcribe the DNA base sequence to the RNA intermediate, which is ultimately translated to protein. Though the contribution of each subunit in RNA synthesis has been clearly elucidated, the role of the smallest ω-subunit is still unclear despite several studies. Recently, a study on a dominant negative mutant of rpoZ has been reported in which the mutant was shown to render the RNA polymerase defective in transcription initiation (ω6, N60D) and gave an insight on the function of ω in RNA polymerase. Serendipitously, we also obtained a silent mutant, and the mutant was found to be lethal during the isolation of toxic mutants. The primary focus of this study is to understand the mechanistic details of this lethality. Isolated ω shows a predominantly unstructured circular dichroism profile and becomes α-helical in the enzyme complex. This structural transition is perhaps the reason for this lack of function. Subsequently, we generated several silent mutants of ω to investigate the role of codon bias and the effect of rare codons with respect to their position in rpoZ. Not all silent mutations affect the structure. RNA polymerase when reconstituted with structurally altered silent mutants of ω is transcriptionally inactive. The CodonPlus strain, which has surplus tRNA, was used to assess for the rescue of the phenotype in lethal silent mutants.