Turn-on of Inactive Genes by Promoter Recruitment in ESCHERICHIA COLI: Inverted Repeats Resulting in Artificial Divergent Operons

We have characterized two rearrangements consisting of inverted repeats of the argE gene. The promoters (p) of argE and of argCBH face each other over an internal operator. The rearrangements were obtained as reactivations of argE in a strain harboring an argEp deletion on a lambda darg prophage. In both cases the repeat included argE and argCBHp on either side of a unique sequence; the result is a divergent operon in which each copy of argCBHp reads into the adjacent argE repeat. In one case, the pair of repeats adjoins the silent parental gene, forming a triplication (comes from leads to comes from). The other rearrangement consists of a single argE palindrome, but the whole prophage is rearranged into an inverted repeat, analogous to certain lambda dv's. Both structures could be explained by breakage of a replication fork passing argE and by inaccurate rejoining of strands. The lambda dv-like rearrangement would result from breakage at both replication forks of a phage or prophage replicating during transient release of immunity. The triplication would imply breaking of a chromosomal replication fork, formation of a cyclic intermediate by recombination between the daughter duplex molecules and reinsertion into the parental argE gene. Formation of a triplication by replication errors involving appropriate strand switchings and branch migrations can not be excluded however.