FIG 1.

Cre recombinase excises a fluorescence cassette in E. coli. (A) Schematic representation of Cre recombinase strategy for use in Chlamydia spp. Cre expression is used to excise the GFP and β-lactamase resistance (Res) reporter genes when flanked by upstream (US) and downstream (DS) loxP sites, leaving behind a loxP scar sequence. The resulting locus is shown and contains one remaining loxP site. (B) Schematic of pSU-CRE plasmid for conditional expression of Cre in C. trachomatis. (C) E. coli colonies expressing ploxP-GFP and lacking (pSUmC) or expressing (pSU-CRE) Cre recombinase. Colonies were imaged with bright-field and fluorescence microscopy. (D) Excision of the reporter cassette was confirmed for three different transformants (1 to 3) by PCR amplification of the locus with primers annealing within the upstream and downstream flanking regions.