We have cloned and characterized a fragment of the Escherichia coli chromosome spanning glnA, the structural gene for glutamine synthetase (L-glutamate:ammonia ligase (ADP-forming), EC 6.3.1.2]. The fragment also carriers glnG, whose product is necessary for regulation of glnA expression, and a previously unidentified gene whose function we have not discovered. Transcription of glnA and the newly identified gene occurs divergently from a region between the two genes. Transcription of glnA proceeds toward glnG, which is transcribed in the same direction. A region of DNA between glnA and glnG contains genetic information whose loss may result in the inability to reduce expression of glnA and other operons in response to ammonia (the GlnC phenotype).