Nucleic Acids in Chemistry and Biology

It results from (i) a combination of the relative yields for their photochemical formation (approx.
100:25:1), (ii) the capability of DNA polymerase to read through the different lesions, (iii) the errors that
may result and (iv) the ability of enzymes to repair such lesions. It appears that bacterial replication past
cyclobutane dimers is rather accurate whereas bypass of T(6-4)T products is highly mutagenic and leads
to T→C transitions (see above). In yeast, Py<>Py cyclobutane dimers are the most mutagenic lesion. In
mammalian systems, such as hamster cells, GC→AT transitions predominate (50%) with large amounts
of transversions (23%) and tandem and non-tandem double mutations (20%). Cytosine-to-thymine transi-
tion mutations are generally formed through deamination of C<>T or T<>C dimers to U<>T or T<>U
dimers, followed by monomerization to U, and not by deamination of a cytosine photohydrate.
In general, it now appears that:

 the (6-4) photoproduct is more mutagenic than Py<>Py while T<>T is poorly mutagenic;


 in bacteria the (6-4) product is likely to be the major premutagenic lesion;


 in mammalian cells, the (6-4) product is repaired more quickly than the Py<>Py making the

cyclobutane dimers the major premutagenic lesion;


 in repair-deficient strains, the (6-4) product may be the dominant mutational lesion even though its

photochemical yield is lower than for Py<>Py.

8.8.2 Psoralen–DNA Photoproducts

Psoralensare furocoumarins that have been widely used in the phototherapy of psoriasis and other skin
disorders. They act as photochemical cross-linking agents for DNA.^74 This involves psoralen intercalation
followed by two successive photochemical [22] cycloadditions, which create two cyclobutane linkages.
Thymines are the preferred target so that cross-linking occurs mainly at d(TpA) sites. The products are
predominantly the cis-syn stereoisomers and have an overall S-shape as a result of one thymine being
above and the other below the plane of the psoralen (Figure 8.32).
Psoralen cross-linking of a duplex has been developed for the examination of triple-strand helix formation
in homopurine–homopyrimidine DNA sequences (Section 2.3.6). Psoralen is attached by its C-5 position
to a 5
-thiophosphate on a homopyrimidine undecamer nucleotide. On incubation with DNA which has
the complementary sequence followed by UV irradiation, the two parent strands of the DNA become
cross-linked at a TpA step present at the junction between the duplex and the triplex. The presence of
a neighbouring triplex structure interferes with different stages of psoralen interstrand cross-link (ICL)
processing: (i) the ICL-induced DNA repair synthesis in HeLa cell extracts is inhibited by the triplex
structure; (ii) in HeLa cells, the ICL removal viaa nucleotide excision repair (NER) pathway (Section
8.11.4) is delayed in the presence of a neighbouring triplex; and (iii) the binding to ICL of recombinant
Xeroderma pigmentosumA protein, which is involved in pre-incision recruitment of NER factors, is

Covalent Interactions of Nucleic Acids with Small Molecules and Their Repair 319

O O O

OMe O O O

N

H

N

N

N

H O OO

O

dR

Me dR

Me

H

H

Me

H

H

H

i, ii, iii

OMe

Figure 8.32 Photochemical binding of 4-methyl-8-methoxypsoralen to DNA and isolation of dithymidine
photoproducts. Procedures: (i) DNA; (ii) hn320–400nm; and (iii) H