The free concentration of bacteriophage T4-coded gene 32 (single-stranded DNA binding) protein in the cell is autoregulated at the translational level during T4 infection of Escherichia coli. The control of the synthesis of this protein reflects the following progression of net (co-operative) binding affinities for the various potential nucleic acid binding targets present: single-stranded DNA > gene 32 mRNA > other T4 mRNAs ≥ double-stranded DNA. In this paper we show that the free concentration of gene 32 protein is maintained at 2 to 3 μm, and use the measured binding parameters for gene 32 protein, extrapolated to intracellular conditions, to provide a quantitative molecular interpretation of this system of control of gene expression. These results are then further utilized to define the specific autoregulatory binding sequence (translational operator site) on the gene 32 mRNA as a uniquely unstructured finite binding lattice terminated by elements of secondary structure not subject to melting by gene 32 protein at the autoregulated concentration, and to predict how this site must differ from those found on other T4 messenger RNAs. It is shown that these predictions are fully consistent with available T4 DNA sequence data. The control of free protein concentration as a method of genome regulation is discussed in terms of other systems to which these approaches may apply.
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