Three Tn10 polypeptides were detected by analyzing the proteins synthesized in ultraviolet light-irradiated Escherichia coli cells after infection with lambda::Tn10. One of these polypeptides was the previously identified 36,000-dalton TET polypeptide. The other two had approximate sizes of 25,000 and 13,000 daltons. The syntheses of both the TET polypeptide and the 25,000-dalton polypeptide were inducible by tetracycline in lambda-immune hosts. Similarly, the synthesis of the TET polypeptide was inducible in nonimmune hosts. However, the synthesis of the 25,000-dalton polypeptide was constitutive in nonimmune hosts. An amber mutation in a gene required for tetracycline resistance on lambda::Tn10 was isolated that eliminated the synthesis of the TET polypeptide in sup+ hosts but not the synthesis of the 25,000-dalton or the 13,000-dalton polypeptides. The expression of tetracycline resistance from wild-type Tn10 was found to be anomalous in E. coli strains carrying the amber suppressors supD, supE, and supF. In general, strains containing these nonsense suppressors were less resistant to tetracycline.
Plasmids found in six strains of Thiobacillus ferrooxidans were mapped and compared in an effort to detect the origin of replication. Four strains yielded an identical 9.8-kb plasmid, pTFI91. Restriction mapping and Southern blot hybridization analysis were used to confirm this finding. Dissimilar plasmids found in two other strains contained a conserved 2.2-kb SacI region common to pTFI91. DNA sequence analysis of this region showed structural features common to bacterial plasmid replicons. A comparison of the pTFI91 origin with those of T. ferrooxidans pTF-FC2 and other broad host range vectors did not show significant homologous DNA sequences. To verify the replication function, a chloramphenicol acetyl transferase marker gene was ligated at the unique sites of pTFI91, and the plasmid was transformed into Escherichia coli DH5 alpha cells but no transformants were identified. To test the replication of pTFI91 independent of DNA polymerase I in E. coli, different restriction fragments of pTFI91 were cloned into pHSG398 (Cmr, ColEI origin) and transformed into the polA1 mutant SF800, but chloramphenicol-resistant transformants were not detected. Electrotransformation of T. ferrooxidans TFI-70 and Pseudomonas putida ATCC 19151 also failed to yield transformants. The results suggested that the pTFI91 plasmid replicon does not function either in E. coli or in P. putida. Since pTFI91 contains the same origin of replication as other plasmids in several other T. ferrooxidans strains, this replicon may be commonly distributed in T. ferrooxidans.
Recombination between the IS50 sequences that flank Tn5 has been found to occur readily after the transformation of E. coli recA strains with a plasmid that contains direct repeats of these sequences. Analysis of mutants of IS50 indicates that the polypeptide encoded by IS50 that is required for transposition is also required for the recombination. Surprisingly, the mechanism of recombination appears to be similar to general homologous recombination rather than site-specific recombination. This IS50 recombination system may be responsible for resolving transient cointegrate structures containing direct repeats of IS50 or Tn5 during the transposition of IS50 and Tn5.
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