SOS mutagenesis results from up-regulation of translesion synthesis

Irradiation of DNA with ultraviolet light generates a variety of photolesio ns. Among them, are cyclobutane pyrimidine dimers (CPD) and (6-4) photoprod ucts blocking lesions that interfere with DNA replication if left unrepaire d. In addition to efficient pre-replicative excision repair mechanisms, cel ls have evolved damage tolerance pathways enabling them to replicate lesion -containing DNA molecules either by directly replicating through the damage d base (translesion synthesis, TLS) or by employing the locally undamaged c omplementary strand thus avoiding the lesion (damage avoidance pathways, DA ). Using double-stranded vectors with a single T(6-4)T UV lesion and a stra nd segregation analysis (SSA), we have measured the relative utilization of the two tolerance pathways (TLS and DA) in Escherichia coli. During the SO S response the error-prone TLS pathway is strongly stimulated (approximate to 20-fold) at the expense of the error-free DA pathways. Thus, up-regulati on of TLS may turn out to be a general property of the SOS response; a simi lar conclusion was previously reached with the frameshift-inducing N-2-acet ylaminofluorene adduct. Therefore, as far as its contribution to damaged DN A replication is concerned, the SOS response appears to be an induced mutat or state rather than a survival strategy. Depending on the base inserted op posite the lesion, TLS can be error-free or mutagenic. In a wild-type strai n, both forms of TLS are increased to a similar extent during the SOS respo nse. In contrast, in a Delta umuDC strain induction of TLS is totally aboli shed, demonstrating that the UmuDC proteins usually thought to be specifica lly involved in mutagenesis facilitate the recovery of both error-free and mutagenic replication intermediates in vivo. (C) 1999 Academic Press.