Translation initation at the root of the cold-shock translational bias

Research carried out in the last two decades has shown that all living organisms, from bacteria to mammals, have evolved mechanisms to cope with the effects caused by a sudden temperature downshift (cold-shock). Following cold-stress, the mesophilic bacterium Escherichia coli enters an acclimation phase during which cell growth stops for 3–6 hours, while bulk gene expression is drastically reduced, and a set of at least 26 well characterized cold-shock genes is selectively and transiently expressed (Yamanaka, 1999; Gualerzi et al., 2003). The proteins synthesized during the cold-acclimation phase are somewhat artificially classified into early and late cold-shock proteins, and likewise early and late cold-adapted proteins accumulate in the lag phase that precedes the resumption of cell division and growth at low temperature (Figure 1A). Overall, the present perception is that the main purposes of the proteins synthesized during cold adaptation and in cold-adapted cells are: (i) to deal with unfavorable secondary structures of nucleic acids induced/stabilized by the cold, which are expected to hinder basic functions such as transcription, ribosome assembly, and translation; (ii) to oppose the cold-shock-induced decrease in membrane fluidity; (iii) to accumulate sugars which are protective against low temperature, such as trehalose; (iv) to assist protein folding at low temperatures (Graumann and Marahiel, 1998; Phadtare et al., 1999; Gualerzi et al., 2003; Weber and Marahiel, 2003).