BISULFITE INDUCES TANDEM DOUBLE CC-]TT MUTATIONS IN DOUBLE-STRANDED DNA .2. KINETICS OF CYTOSINE DEAMINATION

Deamination of cytosine to uracil in double-stranded DNA (ds DNA) by s odium bisulfite has been monitored with a sensitive genetic assay. In this system, reversion of a mutant in the lacZ alpha gene coding seque nce of bacteriophage M13mp2 C141 was detected by employing an ung bact erial strain defective in the enzyme uracil glycosylase. Within the 4- base target, it is possible to measure the rates of induction of C --> T, C --> A, C --> G, and CC --> TT mutations in DNA that has been inc ubated at physiological temperature and pH and then transfected into u ng+ and ung- E. coli cells, respectively, for amplification and detect ion of the mutation. For concentrations of bisulfite from 1 to 50 mM, the reversion frequency in ung- cells increased linearly with time of incubation. The most interesting features of the bisulfite reaction we re as follow: (1) Mutations were reduced 5-fold in ung+ cells, indicat ing ung is involved in repair of bisulfite-treated transforming DNA. ( 2) Sequencing of 157 revertants revealed that C --> T and tandem CC -- > TT transition mutations comprised 100% of the mutations scored. (3) A unique finding was that, at the highest concentrations and longest i ncubation times, almost every mutant obtained in ds DNA exposed to bis ulfite was found to be a CC --> TT tandem double mutation. (4) The hig h frequency of tandem double mutants is inconsistent with two random, independent mutational events and, coupled with the observed ung depen dence, lends support to the concept of catalytic deamination, wherein bisulfite induces deamination in contiguous cytosines by a concerted m echanism. (5) Equations were derived for calculating the rate constant per site based on the data from DNA sequencing. Mutational rate const ants at four cytosines in the ds DNA target at 37-degrees-C in 10 mM s odium bisulfite, 10 mM Hepes, pH 7.4 varied from 0.06 x 10(-10) s-1 to 0.35 x 10(-10) s-1 and showed a sequence context effect. A cytosine b ordered on both sides by cytosine residues exhibited a mutation rate w hich was twice as great as a cytosine having only one nearest-neighbor cytosine. (6) In a tandem double CC --> TT mutation, there was a 4-5 order of magnitude increase in mutational rate constant of the second genetic event. These findings show that tandem double mutations, which have traditionally been ascribed to UV damage, can also be caused by chemical damage.