Stress-induced modification of <i>Escherichia coli</i> tRNA generates 5-methylcytidine in the variable loop

There has been recent interest in trying to understand the connection between transfer RNA (tRNA) posttranscriptional modifications and changes in-cellular environmental conditions. Here, we report on the identification of the modified nucleoside 5-methylcytidine (m⁵C) in <i>Escherichia coli</i> tRNAs. This modification was determined to be present at position 49 of tRNA Tyr-QUA-II. Moreover, m⁵C levels in this tRNA are significantly elevated under high reactive oxygen specieis (ROS) conditions in <i>E. coli</i> cells. We identified the known ribosomal RNA methyltransferase rsmF as the enzyme responsible for m⁵C synthesis in tRNA and enzyme transcript levels are responsive to elevated levels of ROS in the cell. We further find that changes in m⁵C levels in this tRNA are not specific to Fenton-like reaction conditions elevating ROS, but heat shock can also induce increased modification of tRNA Tyr-QUA-II. Altogether, this work illustrates how cells adapt to changing environmental conditions through variations in tRNA modification profiles.