The cryptic adenine deaminase gene of Escherichia coli. Silencing by the nucleoid-associated DNA-binding protein, H-NS, and activation by insertion elements.

In Escherichia coli there are two pathways for conversion of adenine into guanine nucleotides, both involving the intermediary formation of IMP. The major pathway involves conversion of adenine into hypoxanthine in three steps via adenosine and inosine, with subsequent phosphoribosylation of hypoxanthine to IMP. The minor pathway involves formation of ATP, which is converted via the histidine pathway to the purine intermediate 5-amino-4-imidazolecarboxamide ribonucleotide and, subsequently, to IMP. Here we describe E. coli mutants, in which a third pathway for conversion of adenine to IMP has been activated. This pathway was shown to involve direct deamination of adenine to hypoxanthine by a manganese-dependent adenine deaminase encoded by a cryptic gene, yicP, which we propose be renamed ade. Insertion elements, located from -145 to +13 bp relative to the transcription start site, activated the ade gene as did unlinked mutations in the hns gene, encoding the histone-like protein H-NS. Gene fusion analysis indicated that ade transcription is repressed more than 10-fold by H-NS and that a region of 231 bp including the ade promoter is sufficient for this regulation. The activating insertion elements essentially eliminated the H-NS-mediated silencing, and stimulated ade gene expression 2-3-fold independently of the H-NS protein.