The MiaA tRNA Modification Enzyme Is Necessary for Robust RpoS Expression in Escherichia coli

The stationary phase/general stress response sigma factor RpoS (σS ) is necessary for adaptation and restoration of homeostasis in stationary phase. As a physiological consequence, its levels are tightly regulated at least at two levels. Multiple small regulatory RNA molecules modulate its translation, in a manner that is dependent on the RNA chaperone Hfq and therpoS 5′ untranslated region. ClpXP and the RssB adaptor protein degrade RpoS, unless it is protected by an anti-adaptor. We here find that, in addition to these posttranscriptional levels of regulation, tRNA modification also affects the steady-state levels of RpoS. We screened mutants of several RNA modification enzymes for an effect on RpoS expression and identified themiaA gene, encoding a tRNA isopentenyltransferase, as necessary for full expression of both anrpoS750-lacZ translational fusion and the RpoS protein. This effect is independent ofrpoS , the regulatory RNAs, and RpoS degradation. RpoD steady-state levels were not significantly different in the absence of MiaA, suggesting that this is an RpoS-specific effect. TherpoS coding sequence is significantly enriched forleu codons that use MiaA-modified tRNAs, compared torpoD and many other genes. Dependence on MiaA may therefore provide yet another way for RpoS levels to respond to growth conditions.