recX, a new SOS gene that is co-transcribed with the recA gene in Escherichia coli.

recX is a small open reading frame located downstream of recA that is conserved in many bacteria. In Escherichia coli, the recX gene (also named oraA) is a 501 bp open reading frame that encodes a predicted basic protein. Transcriptional analysis by Northern blots showed that in E.

DNA polymerases II and V mediate respectively mutagenic (-2 frameshift) and error-free bypass of a single N-2-acetylaminofluorene adduct.

The NarI sequence represents a strong mutation hot spot for -2 frameshift mutations induced by N-2-acetylaminofluorene (AAF), a strong chemical carcinogen. Only when bound to the third (underlined) guanine (5'-GGCGCC-->GGCC) can AAF trigger frameshift mutations, suggesting the involvement of a slipped replication intermediate with a two-nucleotide bulge. While base substitutions induced by UV light or abasic sites require DNA polymerase V (Pol V; umuDC), the AAF-induced -2 frameshift pathway requires DNA polymerase II, the polB gene product.

Sequence-dependent modulation of frameshift mutagenesis at NarI-derived mutation hot spots.

The NarI sequence is known to be the strongest mutation hot spot for induced frameshift mutagenesis. Indeed, a single N-2-acetylaminofluorene (AAF) adduct induces -2 frameshift mutations (5'-GGCGAAFCC--> 5'-GGCC) more than 10(7)-fold over background mutagenesis in Escherichia coli. The mechanism of induction of the frameshift mutation involves a two nucleotide primer-template misalignment event during replication of the adduct-containing sequence.

Cellular strategies for accommodating replication-hindering adducts in DNA: control by the SOS response in Escherichia coli.

The replication of double-stranded plasmids containing a single adduct was analyzed in vivo by means of a sequence heterology that marks the two DNA strands. The single adduct was located within the sequence heterology, making it possible to distinguish trans-lesion synthesis (TLS) events from damage avoidance events in which replication did not proceed through the lesion. When the SOS system of the host bacteria is not induced, the C8-guanine adduct formed by the carcinogen N-2-acetylaminofluorene (AAF) yields less than 1% of TLS events, showing that replication does not readily proceed through the lesion.