Construction of a Cre-recombinase expressing Tetrahymena strain. (A, B) Plasmid maps of pMNMM3 (A) and pMNMM3-HA-cre1 (B). (C, D) Two possible homologous recombination events between the MNMM3-HA-cre1 construct and the Tetrahymena MTT1 genomic locus. Homologous recombination at "MTT1-5'(1)" and "MTT1-3'" integrates both neo5 and the HA-cre1 gene (C), whereas recombination at "MTT1-5'(1)" and "MTT1-5'(2)" integrates only the neo5 cassette into the genome (D). (E) PCR analysis of the CRE556 strain. Genomic DNA from the CRE556 strain was used to amplify the HA-cre1-containing locus (HA-cre1) and wild-type MTT1 locus (MTT1). The positions of the primers are represented by arrowheads in (C).