The quality and quantity of carbohydrate available to the cell is the major limiting-factor for the capacity of energy-consuming nitrogen fixation. In enteric-bacteria, in response to the PTS system, the cAMP receptor protein (CRP) mediates the glucose effect at transcriptional level. Recently, we have observed that CRP can repress a a54-dependent promoter in a cAMP dependent, binding site independent fashion. Direct interaction between CRP and promoter bound a54 RNA polymerase plays an important role in repression (Wang et al. 1998), which may suggest that the CRP-mediated repression effect on a o54-dependent promoter is general.

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