Proton magnetic resonance spectra at 250 MHz were measured as a function of temperature and pH of the three guanosine phosphates. From these data and previously published work the conformational parameters of these compounds were determinated. The phosphate group of Guo-5'-P changes its conformation around the C-O bond and its rotation is relatively slow at 20 degrees. At neutral pD the S conformation is favoured and the N form at acid pD. This conformational change is paralleled by a change in exocyclic rotamer distribution and takes place at the pK of the protonation of the base on N-7. Although correlation appears to exist between the various conformations, notable exceptions exist.
The dam gene of Escherichia coli encodes a DNA methyltransferase that methylates the N6 position of adenine in the sequence GATC. It was stably expressed from a shuttle vector in a repair- and recombination-proficient strain of Bacillus subtilis. In this strain the majority of plasmid DNA molecules was modified at dam sites whereas most chromosomal DNA remained unmethylated during exponential growth. During stationary phase the amount of unmethylated DNA increased, suggesting that methylated bases were being removed. An ultraviolet damage repair-deficient mutant (uvrB) contained highly methylated chromosomal and plasmid DNA. High levels of Dam methylation were detrimental to growth and viability of this mutant strain and some features of the SOS response were also induced. A mutant defective in the synthesis of adaptive DNA alkyltransferases and induction of the adaptive response (ada) also showed high methylation and properties similar to that of the dam gene expressing uvrB strain. When protein extracts from B. subtilis expressing the Dam methyltransferase or treated with N-methyl-N'-nitro-N-nitroso-guanidine were incubated with [3H]-labelled Dam methylated DNA, the methyl label was bound to two proteins of 14 and 9 kD. Some free N6-methyladenine was also detected in the supernatant of the incubation mixture. We propose that N6-methyladenine residues are excised by proteins involved in both excision (uvrB) and the adaptive response (ada) DNA repair pathways in B. subtilis.
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