Induction of a tautomeric shift by 5-bromo-uracil base analogue

(a) 5-bromo-uracil (5BU) is a modified form of thymine (T). (b) In its keto configuration, 5BU has the same base-pairing as T, by two H-bonds with adenine (A). In the tautomeric enol form, three H-bonds are available and it pairs with the alternative purine base, guanine (G). (c) Incorporation of 5BU into dsDNA as a 5BU = A basepair produces either a T = A pair from the strand with the A, or if the 5BU ' has undergone a tautomeric shift,  a 5BU ' = G pair from the strand with the 5BU. In the next round of replication, the mis-incorporated G correctly pairs with a C as usual. With respect to the original T = A basepair, the C = G basepair is a TC transition mutation.

Figure © 2012 TA Brown, Introduction to Genetics (1st ed.); additional text © 2012 by Steven M. Carr