We are studying the mechanism by which E. coli controls protein synthesis and messenger RNA degradation in response to changes in growth rate. Our previous work had shown that an energy source shift-down leads to a decreased rate of translational initiation and a stabilization of general mRNA. The most dramatic consequences of this control is the accumulation of 70S "initiation monosomes". Present studies are directed to the following objectives: 1. The proteins of the initiation monosomes are being studied in order to determine which step of the ordered sequence of initiation factor reactions has been slowed as a result of the shift-down. 2. We are examining the genetic basis for naturally occurring differences betwen E. coli strains with respect to their ability to control translation after a shift-down. 3. We are assessing the role of translational regulation in the physiological control of mRNA inactivation rates. 4. We are trying to determine whether various changes in nucleotide pools which accompany a shift-down have a role in the regulation of translational initiation. 5. We are attempting to identify a (hypothetical) endonuclease with a role in site-specific inactivation of messenger RNA.