Uncharged tRNA, protein synthesis, and the bacterial stringent response.

Uncharged tRNA has been shown in vivo to have an active role both in the stringent response, and in modulating the rate of translational elongation. Both of these effects appear to be mediated by codon-anticodon interactions on the ribosome. Although the involvement of uncharged tRNA in the stringent response was expected from in vitro experiments, it has only recently been confirmed in vivo. Inhibition of translation by cognate uncharged tRNA was not expected, and a model is proposed in which excess uncharged tRNA competes with charged tRNA (in ternary complex) for the 30S component of the ribosomal A site. When uncharged tRNA is in sufficient excess over charged tRNA, interaction of uncharged tRNA with the 50S component of the A site occurs as well, leading to a stringent response. The cell has a continuum of responses to decreasing aminoacyl-tRNA levels: in moderately limited conditions, the proportion of uncharged tRNA increases, and the translation rate is slowed; under more severe limitations, uncharged tRNA provokes a stringent response, with pleiotropic consequences for the cell.