Abstract
An unequal sister chromatid exchange (USCE) in the mouse myeloma cell line MPC-11 between 3' regions of the C gamma 2a and C gamma 2b heavy chain genes results in duplication of the C gamma 2a heavy chain gene and generation of a novel recombination joint. The USCE occurs between (TC)n tracts adjacent to alternating purine-pyrimidine tracts. We have investigated the capacity of both the donor regions and the recombinant product involved in this event to adopt left-handed Z-DNA and intramolecular triplexes. The results of chemical probing with diethylpyrocarbonate and osmium tetroxide at the base pair level demonstrate that under the influence of negative supercoiling the alternating purine-pyrimidine regions of these plasmids can adopt Z-DNA at neutral pH, and the oligopurine.oligopyrimidine (pur.pyr) regions of these regions can adopt intramolecular triplexes at low pH (less than or equal to pH 6.0). At intermediate pH values, mixtures of both structures are present. Increasing the negative superhelical density of the plasmid does not increase the amount of triplex present at neutral pH indicating that the presence of long Z-DNA segments adjacent to pur.pyr tract prevents intramolecular triplex formation. In summary, we conclude that the sequences involved in the USCE can form either an intramolecular triplex in the (TC)n tract or Z-DNA in the alternating purine-pyrimidine tract and that Z-DNA will predominate under physiological conditions. The presence of segments which adopt Z-DNA at a site of USCE suggests that formation of this structure may enhance recombination between adjacent pur.pyr tracts.
Highlights
From the $Department of Cell Biology, Albert Einstein College of Medicine, Bronx, New York 10461 and the ITDepartment of Biochemistry, Schools of Medicine/Dentistry, University of Alabama, Birmingham, Alabama 35294
We conclude that the sequences involved in the unequal sister chromatid exchange (USCE) can form either an intramolecular triplex in the (TC), tract or Z-DNA in the alternating purine-pyrimidine tract and that Z-DNA will predominate under physiological conditions
In this study we have found that the (TC), and (TG), tracts that lie 3’ of the Crab and Cy2a genes which are involved in the USCE event are hyperreactive to a variety of chemical probes in negatively supercoiled plasmids
Summary
We have investigated the capacity of both the donor regions and the recombinant product involved in this event to adopt left-handed Z-DNA and intramolecular triplexes. We conclude that the sequences involved in the USCE can form either an intramolecular triplex in the (TC), tract or Z-DNA in the alternating purine-pyrimidine tract and that Z-DNA will predominate under physiological conditions. The mouse myeloma cell line MPC-11 (IgG2b, K) carries a tandem duplication of the Cy2a gene together with a deletion of a portion of the CySa-associated tandem repeats on the expressed chromosome [1] This duplication has been shown to be the result of an unequal sister chromatid exchange (USCE)’ [1]. These resuIts suggest that intramolecular triplex formation is not likely to be directly involved in in uiuo recombination in this system but instead that Z-DNA may contribute to recombination within the flanking (TC), tracts
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