A switch from translational control to transcriptional control of protein synthesis in mid-exponential growth phase of bacterial cultures. Specific radioimmune labelling of ribitol-dehydrogenase-synthesising polysomes from Klebsiella aerogenes in the presence of heparin.

1. We present evidence suggesting a sudden switch from translational control to transcriptional control of protein synthesis in mid-exponential growth of bacterial batch cultures. At a critical cell density a switch from large to small polysomes occurs during a short period of exponential growth. The profile of specific polysomes engaged in synthesis of a constitutive enzyme, ribitol dehydrogenase, changes at the same point but in an opposite way: a linear profile peaking at monosomes changes to a dome-shaped profile peaking at about 15 ribosomes/mRNA, which persists into late exponential phase despite a gradual reduction in the total polysome population. The switch in the pattern of protein synthesis is exhibited dramatically by changes in the specific activity or ribitol dehydrogenase in cell extracts at different stages of batch culture. In early exponential phase the specific activity of the enzyme is constant, but it begins to rise suddenly, at the same point at which the polysome profiles change, and continues to increase up to the end of exponential phase. This effect is exhibited by the strains of Klebsiella aerogenes that are inducible for (in the presence of the inducer), consitutive for, or superproducers of ribitol dehydrogenase, and it appears to be unrelated to catabolite repression. 2. The above results depend on improved techniques for production of large amounts of bacterial polysomes and the ability to label nascent peptides attached to polysomes very specifically with radioactive antibody to ribitol dehydrogenase. Our success was due to the observation that sodium heparin completely abolishes non-specific interactions of the antibody with the polysomes.