A new method for specific cleavage of megabase-size chromosomal DNA by lambda-terminase

Chromosomal organization of TOX2, a complex locus controlling host-selective toxin biosynthesis in Cochliobolus carbonum

Race 1 isolates of the filamentous fungus Cochliobolus carbonum are exceptionally virulent on certain genotypes of maize due to production of a cyclic tetrapeptide, HC-toxin. In crosses between toxin-producing (Tox2+) and toxin-nonproducing (Tox2-) isolates, toxin production segregates in a simple 1:1 pattern, suggesting the involvement of a single genetic locus, which has been named TOX2. Earlier work had shown that in isolate SB111, TOX2 consists in part of two copies of a gene, HTS1, that encodes a 570-kD cyclic peptide synthetase and is lacking in Tox2- isolates. The genomic structure of TOX2 and the relationship between the two copies of HTS1 have now been clarified by using pulsedfield gel electrophoresis and physical mapping. In isolate SB111, both copies of HTS1 are on the largest chromosome (3.5 Mb), which is not present in the related Tox2- strain SB114. Two other genes known or thought to be important for HC-toxin biosynthesis, TOXA and TOXC, are also on the same chromosome in multiple copies. Other independent Tox2+ isolates also have two linked copies of HTS1, but in some isolates the size of the chromosome containing HTS1 is 2.2 Mb. Evidence obtained with Tox2+ -unique and with random probes is consistent with a reciprocal translocation as the cause of the difference in the size of the HTS1-containing chromosome among the Tox2+ isolates studied here. Physical mapping of the 3.5-Mb chromosome of SB111 that contains HTS1 using rare-cutting restriction enzymes and engineered restriction sites was used to map the chromosome location of the two copies of HTS1 and the three copies of TOXC. The results indicate that TOX2 is a complex locus that extends over more than 500 kb. The capacity to produce HC-toxin did not evolve by any single, simple mechanism.

GCN4 eukaryotic transcription factor/FokI endonuclease-mediated 'Achilles' heel cleavage': quantitative study of protein-DNA interaction

Three proteins, yeast transcription regulatory protein GCN4, M.FokI DNA methyltransferase and R.FokI restriction endonuclease (ENase) were used to attain specific cleavage of DNA at the 18-20-bp GCN4 recognition site. This is a novel version of the 'Achilles' heel cleavage' (AC) technique [Koob et al., Science 241 (1988) 1084-1086]. Since the method employs a class-IIS ENase (R.FokI), which cleaves the DNA outside of its recognition sequence, it leaves the overlapping GCN4-binding intact. Thus, the same GCN4 site can be used in consecutive cleavage reactions. This novel GCN4-IIS-AC technique was applied to study the protein-DNA interaction. Quantitative analysis of the effect of temperature, reaction time, and GCN4 and M.FokI concentrations allowed determination of the GCN4-DNA complex half-life, which was found to be 7 h at 30 degrees C, 18 h at 22 degrees C and over 24 h at 10 degrees C. In addition, conditions for controlled, partial GCN4-IIS-AC digestion of DNA were determined, and applied to the physical mapping of large genomes.

Site-specific cleavage of chromosomes in vitro through Cre-lox recombination

Site-specific recombination systems are useful tools for chromosome engineering in vivo and site-specific DNA cleavage methods have applications in genome analysis and gene isolation. Here, we report a new method to fragment chromosomes in vitro using the Cre-lox site-specific recombination system. Two lox sites were targeted into the 5.7 Mb chromosomes I of Schizosaccharomyces pombe. In vitro recombination between chromosomal lox sites and exogenously provided lox oligonucleotides 'cleaved' the chromosome at the defined lox sequences. Site-specific cleavage of lox sites in the tobacco genome was also demonstrated. This recombination-based cleavage method provides a novel approach for structural and functional analyses of eukaryotic chromosomes as it allows direct isolation of chromosome regions that correspond to phenotypes revealed through Cre-lox mediated chromosome rearrangements in vivo. Moreover, recombination with end-labeled lox oligonucleotides would permit the specific end-labeling of chromosome segments to facilitate the long range mapping of chromosomes.