The GcvA protein both activates and represses the operon and negatively regulates its own transcription. GcvA binds to three sites in the control region and to one site in the control region; each of these binding sites contains the conserved 5bp DNA sequence 5′-CTAAT-3′. This report describes the role this DNA sequence plays in autoregulation and expression of . Through single base-pair mutations, the importance of three of these five basepairs in the autoregulation of expression is shown. Two of the control region mutations described cause a :: fusion to be overexpressed at 9-24 times the wild-type level. The increase in expression is due in part to a complete loss of autoregulation and in part to a GcvA-independent mechanism. One of the mutants was shown by Western blot analysis to increase the intracellular concentration of GcvA. This high level of expression subsequently causes the loss of purine-mediated repression of a :: fusion. However, overexpression of re-established purine-mediated repression of the :: fusion, supporting the model for regulation that suggests the need for a relatively constant GcvA to GcvR ratio for appropriate regulation of expression in response to glycine and purine availability.


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