WEAVER: FIG' 20'38

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UV-INDUCED FORMATION OF PYRIMIDINE
(THYMINE) DIMERS UV LIGHT OF 260 nm LEADS TO
THE FORMATION OF A CYCLOBUTANE RING
BETWEEN ADJACENT BASES WHICH DISTORT THE
PHOSPHO- DIESTER BACKBONE
WEAVER FIG. 20.38
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MISMATCH REPAIR IN E. coli
NEWLY SYNTHESIZED DNA STRAND IS RECOGNIZED BY THE
ABSENCE OF METHYL GROUPS ON GATC SEQUENCES
WEAVER FIG. 20.47
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MODEL OF TRANSLESION SYNTHESIS (TLS)
(a) POL III associated with its ?-clamp
encounters a lesion, such as a modified
nucleotide or thymine dimer, in the template and
stops. Multiple copies of the RecA protein then
bind to the single-stranded DNA downstream from
the lesion. Pol III is released and the bypass
polymerase, Pol V, associates with the site
vacated by Pol III. The binding of Pol V
is dependent on both RecA and the existing
?-clamp. Pol V next synthesizes a 20-nucleotide
patch, after which it dissociates and is
replaced by Pol III. Normal DNA replication then
resumes. As the fidelity of Pol V (10-3 to 10-4)
is considerably less than that of Pol III, base
mismatches may be introduced during TLS that have
to be corrected later by other repair
mechanisms. (b) Successful translesion bypass
requires the patch to be at least 5 nucleotides
long. If the patch is less than 5 nucleotides,
the distortion is sensed by the returning Pol
III and it is degraded by the enzymes exonucleas
e activity.
From Fujii Fuchs (2004), EMBO J. 23, 4342-4352
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