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Summary Transcriptional regulation is the key to ensuring that proteins are expressed at the proper time and the proper amount. In Escherichia coli , the transcription factor cAMP receptor protein (CRP) is responsible for much of this regulation. Questions remain, however, regarding the regulation of CRP activity itself. Here, we demonstrate that a lysine (K100) on the surface of CRP has a dual function: to promote CRP activity at Class II promoters, and to ensure proper CRP steady state levels. Both functions require the lysine's positive charge; intriguingly, the positive charge of K100 can be neutralized by acetylation using themore » central metabolite acetyl phosphate as the acetyl donor. We propose that CRP K100 acetylation could be a mechanism by which the cell downwardly tunes CRP‐dependent Class II promoter activity, whilst elevating CRP steady state levels, thus indirectly increasing Class I promoter activity. This mechanism would operate under conditions that favor acetate fermentation, such as during growth on glucose as the sole carbon source or when carbon flux exceeds the capacity of the central metabolic pathways.« less