EXCISION AND TRANSPOSITION OF TN5 UPON INSERTION IN THE HHA GENE OF ESCHERICHIA-COLI

We report the occurrence of Tn5 secondary transpositions in Escherichi a coli HB101 hha::Tn5(pANN202-312). Tn5-induced hha mutants (hemolysin overproducers) segregated at high frequency either hemolysin-negative clones or clones showing the parental hemolytic phenotype (reduced he molysin production). Secondary transpositions of Tn5 appeared to be re sponsible for the phenotypes of both types of derivatives. The hemolys in-negative clones no longer harboured Tn5 in the hha gene, but rather Tn5 was in the hly genes of the hemolytic plasmid pANN202-312. The de rivatives that recovered the parental hemolytic phenotype contained a single copy of Tn5 in a chromosomal location different from that of hh a. Both kinds of transpositional events appeared to restore the functi on of the hha gene.