engleski

Biological cost of the 16S rRNA modification conferred by aminoglycoside resistance methyltransferase Sgm

Sgm methyltransferase from actinomycete Micromonospora zionensis is a member of the Arm family of enzymes that confer high level resistance to aminoglycoside antibiotics. Their mode of action is modification of the A-site of the ribosome by methylation of the nucleotide G1405 at the position N-7. Structure and function of the A-site of the ribosome is highly conserved throughout all three kingdoms of life and so are the nucleotide sequence and posttranscriptional modification content of the 16S rRNA contained within. Nucleotide G1405 is located in the upper part of helix 44 in 16S rRNA, in the decoding center of the ribosome. In the vicinity of G1405, there are several nucleotides modified by housekeeping methyltransferases, such as m4Cm1402, m5C1407 and m3U1498. Here we propose that the addition of another methyl group in this highly evolutionary conserved region in the ribosome could have its biological cost. We investigated how the presence of the Sgm enzyme affects the exponential growth of E. coli cells in various media and temperature conditions and the ability of bacteria to compete with the cells not expressing the enzyme. To see if m7G1405 methylation influences the ribosome assembly, we compared ribosomal profiles of cells carrying Sgm methyltransferase with the wild type cells, both in the presence and the absence of kanamycin or gentamicin. Our results revealed that E. coli cells expressing Sgm enzyme grow slightly slower compared with the non expressing cells. Moreover, cells without Sgm outcompeted Sgm expressing cells when grown together, suggesting that the presence of the Sgm enzyme is advantageous only in the presence of antibiotics. Since the ribosome assembly was not influenced by the presence of Sgm enzyme, we propose that the biological cost of m7G1405 methylation in the ribosomal A-site could be caused by the hindrance of the protein synthesis process.

aminoglycoside antibiotics; 16S rRNA methylation; methyltransferase Sgm

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