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Covalently closed circular plasmid DNA was treated with three reactive derivatives of 2‐acetylaminofluorene: N‐acetoxy‐N‐2‐acetylaminofluorene (N‐Aco‐AAF), its 7‐iodo derivative (N‐Aco‐ AAIF) and N‐hydroxy‐N‐2‐aminofluorene (N‐OH‐AF), and tested as substrates for the Escherichia coli uvrABC endonuclease and for transformation frequencies on wild‐type, uvrA, recA, uvrArecA and polA mutant strains. The uvrABC endonuclease reacted with all three substrates with high efficiency, implicating this enzyme in the repair of DNA containing all three types of adducts. However, only AAF‐ and AAIF ‐DNA showed greatly reduced survival on uvrA mutants (five adducts/lethal hit) relative to wild‐type (20 adducts/lethal hit). AF‐DNA survived equally well on uvrA mutant and wild‐type cells, and at a much higher level of modification (60 adducts/lethal hit). A mutation in recA had only a minor effect on the survival of either DNA. The polA mutation reduced the survival of the AAF‐treated DNA to the same extent as the uvrA mutation (five adducts/lethal hit). Also AF‐DNA showed reduced survival on polA mutant cells versus wild‐type. However, many more adducts (20/lethal hit) were tolerated than for AAF‐DNA, indicating that AF lesions in the template do not efficiently block replication of DNA.

The PPR1 gene encodes the positive regulator of the URA1 and URA3 genes in yeast. Its transcription product is a 2.9‐kb polyadenylated RNA with an extremely short half‐life of 1 min. The induced or non‐induced cell contains approximately 0.1 molecules of PPR1 RNA, a constitutive level which is not altered by changing the number of structural genes to be regulated. The DNA sequence of a 399‐bp AccI‐Bg/II fragment including 180 nucleotides of the 5′‐flanking region of the gene PPR1 has been determined. By S1 mapping we present evidence that the 5′ non‐coding region of the PPR1 mRNA is heterogeneous in length. The 5′ termini of the different RNAs map 20, 36, 45 and 50 nucleotides upstream from the translation start codon. The sequence indicates that the only open translation phase begins with an AUG codon that is preceded by two out‐of‐frame AUG triplets. This particular structure of the 5′‐terminal sequence of the transcripts of PPR1 is discussed in relation to both their stability and translational efficiency.