(Colleaux et al., 1986). Similar results were obtained with 100% of the constructs in the orientation (-). It is unlikely that the DNA sequence of the insert is deleterious for the host cell, since the insert can be integrated in orientation (-) and since E. coli can propagate pUC57 carrying I-SceI gene. Thus, these results suggest that (1) I-SceI gene is expressed when inserted in orientation (+) in pIJ6902 and (2) the produced mRNA or protein is deleterious for the host cell. A trivial hypothesis to explain this deleterious effect is that the I-SceI protein could cut the host chromosome through its endonuclease activity. However, I-SceImeganuclease targets an 18 bp recognition sequence which theoretically appears once in 70 billion bases of a random sequence and which is absent from 7 sequenced E. coli genomes. An alternative hypothesis for I-SceI toxicity in E. coli is that it could bind and inhibit a trans factor or aspecifically interact with DNA. The fact that I-SceI gene is expressed when inserted in orientation (+) implies that the gene is transcribed from the tipA promoter or from another promoter sequence upstream NdeI cloning site. Yet, no known sigma factors binding elements of E. coli has been identified upstream the cloning site. Furthermore, the cloning site is flanked by two strong transcriptional terminators, excluding any transcription initiation of I-SceI gene from another open reading frame of pIJ6902.