Recently we have reported about a novel stress response protein (pY or RaiA) associated with Escherichia coli ribosomes that inhibits translation at the aminoacyl-tRNA binding stage. Here we show that leucine misincorporation during in vitro poly(U) translation is inhibited by this protein much stronger than the incorporation of phenylalanine. The miscoding counteraction by RaiA is especially strong at the concentrations of magnesium ions close to those observed in vivo and diminishes at higher magnesium concentrations. The results obtained suggest that the anti-miscoding activity of RaiA could be the main function of the protein, rather than the inhibition of translation. The role of the protein in adaptation of cells to environmental stress is discussed. (C) 2004 Published by Elsevier Inc.