The scales of experience: Introduction to the special issue Experiencing the global environment

The Scales of Experience introduces the special issue Experiencing the Global Environment by focusing on three dimensions of the theme that are reflected to various degrees in the constitutive essays. First, the introduction highlights the links between the epistemological and political contexts of the historical constitution and development of the global environment (or global environments) in the earth and environmental sciences from the late nineteenth century to today. Second, it argues for a historical approach to the complex concept of scientific experience, whose mutable and contingent qualities are demonstrated by the contributions to the special volume. Lastly, the introduction presents one of the central issues to be tackled by the essays to follow: the development – and, at times, the failure – of strategies and technologies to bridge the seemingly incommensurate gulf between individual, localized experience and the all-encompassing scale of the global environment.     

The E-site story: the importance of maintaining two tRNAs on the ribosome during protein synthesis

In the sixties James Watson suggested a twosite model for the ribosome comprising the P site for the peptidyl transfer RNA (tRNA) before peptide-bond formation and the A site, where decoding takes place according to the codon exposed there. In the eighties a third tRNA binding site was detected, the E site, which was specific for deacylated tRNA and turned out to be a universal feature of ribosomes. However, despite having three tRNA binding sites, only two tRNAs occupy the ribosome at a time during protein synthesis: at the A and P sites before translocation (PRE state) and at the P and E sites after translocation (POST state). The importance of having two tRNAs in the POST state has been revealed during the last 25 years, showing that the E site contributes two fundamental features: (i) the fact that incorporation of a wrong amino acid is not harmful for the cell (only 1 in about 400 misincorporations destroys the function of a protein) stems from the presence of an E-tRNA; (ii) maintenance of the reading frame is one of the most remarkable achievements of the ribosome, essential for faithful translation of the genetic information. The presence of the POST state E-tRNA prevents loss of the reading frame. Received 14 March 2006; received after revision 8 June 2006; accepted 4 August 2006