Previous studies have shown that there is a deoxyribonucleic acid (DNA) segment, of length 1.3 kb and denoted as the alphabeta sequence, which occurs twice on the F plasmid at corrdinates 93.2 to 94.5/OF kb and 13.7 to 15.0F kb. In the present investigation, heteroduplexes were prepared between a phage DNA carrying the insertion sequence IS3 and suitable F-prime DNAs. The hybrids formed show that IS3 is the same as alphabeta. This result plus previous studies support the view that: (i) the insertion sequence IS2 and IS3 occur on F and, in multiple copies, on the main bacterial chromosome of Escherichia coli K-12; and (ii)these IS sequences on the main bacterial chromosomes are hot spots for Hfr formation by reciprocal recombination with the corresponding sequences of F
