The genetic structure and functional organization of a Bacteroides conjugative transposon (CTn), CTn341, were determined. CTn341 was originally isolated from a tetracycline-resistant clinical isolate of Bacteroides vulgatus. The element was 51,993 bp long, which included a 5-bp coupling sequence that linked the transposon ends in the circular form. There were 46 genes, and the corresponding gene products fell into three major functional groups: DNA metabolism, regulation and antibiotic resistance, and conjugation. The G؉C content and codon usage observed in the functional groups suggested that the groups belong to different genetic lineages, indicating that CTn341 is a composite, modular element. Mutational analysis of genes representing the different functional groups provided evidence for the gene assignments and showed that the basic conjugation and excision genes are conserved among Bacteroides spp. A group IIA1 intron, designated B.f.I1, was found to be inserted into the bmhA methylase gene. Reverse transcriptase PCR analysis of CTn341 RNA showed that B.fr.I1 was functional and was spliced out of the bmhA gene. Six related CTn-like elements were found in the genome sequences of Bacteroides fragilis NCTC9343 and Bacteroides thetaiotaomicron VPI5482. The putative elements were similar to CTn341 primarily in the tra and mob regions and in the exc gene, and several appeared to contain intron elements. Our data provide the first reported sequence for a complete Bacteroides CTn, and they should be of considerable benefit to further functional and genetic analyses of antibiotic resistance elements and genome evolution in Bacteroides.
We have demonstrated that targeted mutagenesis can be accomplished in maize plants by excision, activation, and subsequent elimination of an endonuclease in the progeny of genetic crosses. The yeast FLP/FRT site-specific recombination system was used to excise and transiently activate the previously integrated yeast I-SceI homing endonuclease in maize zygotes and/or developing embryos. An artificial I-SceI recognition sequence integrated into genomic DNA was analyzed for mutations to indicate the I-SceI endonuclease activity. Targeted mutagenesis of the I-SceI site occurred in about 1% of analyzed F1 plants. Short deletions centered on the I-SceI-produced double-strand break were the predominant genetic lesions observed in the F1 plants. The I-SceI expression cassette was not detected in the mutant F1 plants and their progeny. However, the original mutations were faithfully transmitted to the next generation indicating that the mutations occurred early during the F1 plant development. The procedure offers simultaneous production of double-strand breaks and delivery of DNA template combined with a large number of progeny plants for future gene targeting experiments.
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