Suppression of a dnaA46 mutation by integration of plasmid R100.1 derivatives in the termination region of chromosome replication in E. coli results in medium dependence, the suppressed bacteria being sensitive to rich medium at 42 degrees C. Derivatives of such bacteria have been selected for growth at 42 degrees C in rich medium and we have analyzed representatives of the most frequently observed type: bacteria displaying, once cured of the suppressor plasmid, both rich-medium sensitivity and temperature sensitivity. We found, in all cases, that the chromosome had undergone a major inversion event between two inverted IS5's. One is located at 29.2 min on the chromosome map and the other at either one of two positions between 69 and 80 min. The consequences of such inversions for cell growth are discussed. Some of them result from the fact that the replication terminator T2 is located, in inverted chromosomes, close to oriC in the orientation which allows its functioning as a terminus (de Massy et al. in press). Our observations allow an estimation of the frequency of inversions arising from recombination between pairs of inverted chromosomal IS, which could be as high as 10(-2) per cell per generation. We also found that inversion reversal occurs frequently after Hfr conjugational transfer of one of the IS5's, in its wild-type location. This led us to propose a new mechanism of recombination, in which the incoming DNA strands serve as guides to favor recombination between the resident sequences.