MECHANISMS OF SPONTANEOUS MUTAGENESIS IN ESCHERICHIA COLI

WE HAVE OBTAINED A SPECTRUM OF MUTATIONS OCCURRING BOTH IN THE SUPF GENE ON THE PLASMID AND IN THE TONB GENE ON THE CHROMOSOME. TWENTY SUPF MUTATIONS WERE ANALYZED. BASE CHANGES COMPRISED 55% OF EVENTS, AND TRANSVERSIONS WERE PREDOMINANT. IS ELEMENTS CONTRIBUTED 30% OF THE EVENTS. ONE OF THE TWO DELETIONS WAS ASSOCIATED WITH THE PRESENCE OF 10 BP DIRECTLY REPEATED SEQUENCES, AND A SECOND ONE DID NOT HAVE REPEATS AT THE JUNCTION. A FRAMESHIFT RESULTED FROM THE LOSS OF C IN A RUN OF 5 C RESIDUES. SIX OUT OF THE 7 TONB MUTATIONS OBTAINED FROM THE WILD-TYPE E. COLI WERE IS ELEMENT INSERTIONS WHICH WERE CLUSTERED WITHIN A 100 BP 5' REGION OF THE GENE INCLUDING THE PROMOTER REGION. THE REMAINING MUTATION WAS A DELETION AND DID NOT HAVE REPEATS AT THE JUNCTION. IN THE RECA56 STRAIN, ALL THE TONB MUTATIONS (6 CASES) WERE CAUSED BY THE IS INSERTIONS THAT OCCURRED DISPERSED THROUGHOUT THE GENE. THERE MAY EXIST AN IS INSERTION "HOT SPOTS" IN TONB. THESE RESULTS PROVIDE DIRECT INFORMATION CONCERNING THE NATURE OF SPONTANEOUS MUTATION ARISING IN E. COLI.