Site of unequal sister chromatid exchange contains a potential Z-DNA-forming tract

Previous studies from this laboratory have provided evidence that the duplication of the IgG2a heavy chain constant region gene (C gamma 2a) in the murine myeloma cell line MPC-11 occurred via unequal sister chromatid exchange. We now report the determination of the DNA sequences of the germ-line regions in which this exchange has occurred. The two donor sequences have long regions of virtual identity. Furthermore, both chromatids contain stretches of T-C and T-G dinucleotides; the latter has the potential to form Z-DNA. Localization of the actual breakpoints via genomic Southern blot analysis suggests a novel mechanism for the recombination and strongly implicates the simple sequence in the process.