Ribosomal Subunits Synthesis and Assembly in Cold-Shocked and Cold-Adapted Escherichia coli cells

To ensure the correct assembly of ribosomal subunits, ribosomal RNA (rRNA)and ribosomal protein (r-protein) synthesis must be highly coordinated. In this work we have analyzed the in vivo synthesis of r-RNA and its ncorporation into ribosomal subunits of Escherichia coli cells following cold-stress and during cold-acclimation. Our results show that rRNA synthesis does not stop after the abrupt downshift of temperature but its rate of ynthesis is dramatically reduced in the early acclimation phase. In the late acclimation phase the rate of rRNA transcription increases again to a level which is compatible with cell growth and division at low temperature. After cold-shock all seven rrn operons present in E. coli were found to be active but they were found to contribute differently to ribosome assembly. Furthermore, the P1 and P2 tandem promoters of each rRNA operon proved to be differently affected by temperature downshift. Overall the present data indicate that the contribution of each rrn operon to ribosome assembly depends upon the environmental and growth conditions. Under normal conditions of growth, correct ribosomal subunit assembly requires that ribosomal (r) proteins and RNA are synthesized in a highly coordinated temporal and stoichiometric manner. In this work we demonstrate that the coordination between r-protein and rRNA synthesis is lost in cells subjected to cold shock since following this stress, synthesis of r-proteins continues at a faster rate than rRNA transcription. Furthermore, it has been observed that some r-proteins of the small subunit (i.e. S3, S5 and S7), all located in the head of the subunit, are posttranslationally modified only during cellular response to cold stress and that these modified proteins are not present in polysomal ribosomes. Thus, these modified r-proteins seem to define an ntermediate step in small subunit assembly following cold shock