The proteins from
E. coli K-12 and
E. coli B differ by 6 amino acid alterations
[27]. Ada can be also be activated as a transcriptional regulator through a direct methylation by MMS and methyl iodide
[28, 29, 30].
Purified Ada is cleaved by cellular proteinases into two smaller polypeptides derived from the N-terminal and C-terminal halves of the protein
[31]. The C-terminal 178 amino acid domain of Ada (from
E. coli B) has been purified and crystallised; in this structure Cys-321 is buried suggesting that conformational change is required for contact with the methyl group of a DNA molecule
[32]. N-Ada (purifed from
E. coli B) binds a zinc molecule which is tetrahedrally coordinated to 4 cysteine residues (Cys38, Cys42, Cys69 and Cys72); zinc is required for correct protein folding
[33, 34, 35].
Ada is autoregulated and regulates transcription of the genes encoding |FRAME: EG11222-MONOMER "AlkA"|, |FRAME: EG10037-MONOMER "AlkB"| and |FRAME: CPLX0-7691 "AidB"| proteins
[1, 6, 8, 29, 36, 37, 38, 39] The mechanism of transcription regulation by Ada varies depending on the target promoter
[4, 7, 14, 40] (and see review by
[41]. Transcription activation by Ada is controlled by a methylation dependent electrostatic switch whereby methylation of Cys38 enhances DNA binding by reducing the electrostatic repulsion between N-Ada and the sugar-phosphate backbone of DNA
[3, 42].
Undamaged cells produce, on average, one Ada molecule per generation; some cells have no Ada molecules and thus cannot trigger the adaptive response. The stochastic activation of Ada results in subpopulations that do not efficiently repair alkylation damage and accumulate mutations
[43].
We have modified the reported length of the binding site of this protein to 13 bp, according to the proposal by Teo
et al [11]. Other authors have proposed different lengths and consensus sequences for Ada
[5, 9], but Nakamura
et al showed that deletions in this consensus sequence (AAANNAAAGCGCA) decrease the activity of β-galactosidase
[6].
Based on a quantitative model, it was shown that stochastic expression of Ada is responsible for creating stark cellular heterogeneity in the adaptive response
[44]
ada:
adaptive response to alkylation damage
Reviews:
[45, 46, 47, 48, 49, 50].
Related reviews:
[51, 52]