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The structure of cruciforms in supercoiled DNA: probing the single‐stranded character of nucleotide bases with bisulphite.
Author(s) -
Gough G.W.,
Sullivan K.M.,
Lilley D.M.
Publication year - 1986
Publication title -
the embo journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 7.484
H-Index - 392
eISSN - 1460-2075
pISSN - 0261-4189
DOI - 10.1002/j.1460-2075.1986.tb04195.x
Subject(s) - cruciform , biology , cytosine , dna , microbiology and biotechnology , character (mathematics) , nucleotide , genetics , physics , gene , materials science , geometry , mathematics , composite material
The single‐stranded character of cytosine bases in three cruciform structures has been assessed by an examination of reactivity towards sodium bisulphite. Unpaired cytosine residues undergo deamination at C4 to give deoxyuracil, and propagation in an ung Escherichia coli host results in C‐G‐‐‐‐T‐A transition mutations, detectable by restriction cleavage or sequence analysis. Very high frequencies of such mutations have been found at cruciform loops, confirming their unpaired character, with almost zero background mutation frequencies elsewhere. A low level of modification was observed at the four‐way junction of a cruciform. The results indicate that the optimal cruciform loop size is four bases, with loose ‘breathing’ at the first base pair at the top of the cruciform stem at 37 degrees C, and little or no opening of base pairs at the four‐way junction.