FIG. 5.
The purE-pepN region of class IIb as a composite of I and IIa. (a) Comparison of classes I, IIa, and IIb. Heavy arrows, direct repeat units; filled circles, 91 bp identical in Rd and C2859, including p, the first 7 bp of pepN; ∗∗∗∗∗, a 97-bp region containing REP sequences; and hicAB, the hic region. (b) Hypothesized origin of class IIb. A horizontally transferred fragment (IIa′) from a class IIa genome is incorporated into a class I genome, facilitated by homology within pepN. X1, homologous exchange between pepN genes of donor and recipient, occurring downstream from X2, a nonhomologous exchange between the REP region of the donor fragment and pepN nt 7 and 8 of the recipient genome.