Site-directed Mutagenesis Analysis Elucidates the Role of 223/227 Arginine in 23S rRNA Methylation, Which Is in 'Target Adenine Binding Loop' Region of ErmSF

Abstract

ErmSF is one of the Erm family proteins which catalyze S-adenosyl-L-methionine dependent modification of a specific adenine residue (A2058, E. coli numbering) in bacterial 23S rRNA, thereby conferring resistance to clinically important macrolide, lincosamide and streptogramin B (MLSB) antibiotics. 222 FXPXPXVXS 230 (ErmSF numbering) sequence appears to be a consensus sequence among the Erm family. This sequence was supposed to be involved in direct interaction with the target adenine from the structural studies of Erm protein ErmC'. But in DNA methyltarnsferase M. Taq I, this interaction have been identified biochemically and from the complex structure with substrate. Arginine 223 and 227 in this sequence are not conserved among Erm proteins, but because of the basic nature of residues, it was expected to interact with RNA substrates. Two amino acid residues were replaced with Ala by site-directed mutagenesis. Two mutant proteins still maintained its activity in vivo and resistant to the antibiotic erythromycin. Compared to the wild-type ErmSF, R223A and R227A proteins retained about 50% and 88% of activity in vitro, respectively. Even though those arginine residues are not essential in the catalytic step, with their positive charge they may play an important role for RNA binding. et al., 1983;Weisblum, 1995;Roberts, 2004