Nucleotide sequence of the traD region in the Escherichia coli F sex factor

The complete nucleotide sequence has been determined of a 3635-bp region, extending from the HpaI site in traT, at F coordinate 90.3 kb, to beyond the end of traD, of the F sex factor plasmid of Escherichia coli K-12. This region contains the C-terminal coding part of traT and the entire traD gene. An open reading frame (ORF) of 2148 bp within the sequence confirms that traD encodes an 81.4-kDa cytoplasmic membrane protein. The TraD protein has several regions with an unusually high pI (greater than 10), suggesting that they may correspond to the DNA-binding domains. Several other ORFs were detected within the region including the gene (ORF1) for a 26.3-kDa protein and ORF2, probably corresponding to traI, which continues to the end of the sequence. An ORF for an 8.5-kDa protein preceded by an excellent promoter and ribosome-binding site is present in the region following traD but on the opposite strand. This promoter is thought to correspond to the major RNA polymerase binding site in this region, implying that traI does not have its own promoter. The lack of a typical terminator following traD and ORF1 and the translational coupling provided by overlapping stop and start codons is consistent with this conclusion.     

Effects of overproduction of superoxide dismutase on the toxicity of paraquat toward Escherichia coli

Gross overproduction of the manganese-containing superoxide dismutase in Escherichia coli, by virtue of a multicopy plasmid bearing the sodA gene, decreases enumeration on paraquat-containing agar plates. This reflects growth inhibition, not lethality, since cells on these plates can be rescued by exclusion of dioxygen. Growth in liquid medium revealed that the control strain adapted to growth in the presence of paraquat more rapidly than did the overproducer. Glucose-6-phosphate dehydrogenase, taken as a representative of the superoxide-inducible soxR regulon, was induced during exposure to paraquat to a much greater extent in the control than in the superoxide dismutase-over-producing strain. These results support the view that overproduction of superoxide dismutase interferes with induction of the soxR regulon and thus prevents a balanced adaptation to the multiple aspects of the toxicity of aerobic paraquat.