Note that: ORF1 = InsA: 91 aa, ORF2' = InsB: 125 aa, ORF1-2 = InsA-B'-InsB: 232 aa. Although insE was originally thought to encode an accessory protein involved in IS1 transposition (Jakowec et al., 1988), recent experiments (Freund and Susskind, 1996) showed that insE is not essential for the formation of IS1 cointegrates. When the transframe protein is efficiently produced, IS1 generates adjacent deletions and minicircle molecules (Sekine et al., 1997). These molecules consist of the entire IS1 sequence and a 5-9 bp sequence which intervenes between the IS1 terminal repeats (IRL and IRR) and corresponds to the original region flanking either IRL and IRR. IS1 is present in about 14% of the Salmonella strains tested but in more of 90% of those of Escherichia coli (Bisercic and Ochman, 1993b). Three other copies of IS1A have been described: IS1E in Escherichia coli W3110 (Umeda and Ohtsubo, 1991; X52537), IS1K in Escherichia coli (F'lac-proB) (Johnsrud, 1979; J01729) and IS1 in Salmonella typhimurium SARA38 (Bisercic and Ochman 1993b). Two IS1 copies, in inverted orientation, are present in the fae (biosynthesis of K88 fimbriae) regulatory region of Escherichia coli G7 (pRI8801), between the FaeA and FaeB regulatory proteins. It is also hypothesized that these IS1 interfered with the K88 biosynthesis regulation (Huisman et al., 1994). Similarly, two IS1 copies were found to flank a duplicated manC-manB region (involved in the GDP-mannose formation) and possibly participating in the antigenic diversity seen in surface polysaccharides of several Escherichia coli strains (Drummelsmith et al., 1997).