FIG. 2.
The basic strategy for allele exchange. Thick and thin lines represent chromosomal and plasmid sequences, respectively. The hatched rectangles indicate the 16S and 23S rRNA genes. The 5S rRNA and tRNA genes are not shown. Stippled and open rectangles represent the ampicillin and chloramphenicol resistance genes, respectively, and closed rectangles the sacB-Kmr cassette. ori indicates the relative position of the ColE1-type replication origin. Broken lines indicate possible crossover sites for a successful allele exchange. In panels B, C, and D, only a part of the chromosome is shown. See Fig. 1 for definitions of the other symbols.