Mutational analysis of basic residues in the N-terminus of the rRNA:m6A methyltransferase ErmC'

Area of Science:

• Molecular Biology
• Biochemistry
• Microbiology

Background:

• Erm methyltransferases are crucial for bacterial resistance to macrolide-lincosamide-streptogramin B (MLS B) antibiotics.
• This resistance mechanism involves the dimethylation of a specific adenine residue within the 23S ribosomal RNA (rRNA).
• The N-terminal region of Erm methyltransferases, particularly its charged residues, is implicated in RNA interaction and catalytic function.

Purpose of the Study:

• To investigate the role of positively charged N-terminal amino acid residues, specifically lysine 4 (K4) and lysine 7 (K7), in the ErmC' methyltransferase.
• To determine the contribution of these residues to RNA binding and/or the catalytic activity of the enzyme.
• To elucidate the precise function of K4 and K7 in the context of antibiotic resistance conferred by ErmC'.

Main Methods:

• Site-directed mutagenesis was employed to generate mutants at the K4 and K7 positions within the ErmC' methyltransferase.
• In vivo assays were conducted to assess the functional impact of these mutations in a cellular context.
• In vitro experiments were performed to directly measure the enzymatic activity and RNA binding capabilities of the wild-type and mutant enzymes.

Main Results:

• Mutational analysis revealed that K4 and K7 residues are not essential for the basic enzymatic activity of ErmC'.
• However, these residues were found to significantly support the catalytic step of the methylation reaction.
• The supporting role is likely achieved by maintaining an optimal conformation of the transition state through interactions with the rRNA phosphate backbone.

Conclusions:

• The N-terminal K4 and K7 residues of ErmC' methyltransferase play a supportive rather than essential role in catalysis.
• These residues are important for stabilizing the enzyme-substrate complex or transition state, thereby enhancing catalytic efficiency.
• Understanding these interactions provides insights into the mechanism of antibiotic resistance and potential targets for drug development.