Sequence homology requirements for intermolecular recombination in mammalian cells

We have examined the homology requirements for intermolecular recombination between plasmids introduced into human, monkey, and bacterial cells. Variable-size-deletion derivatives of the prokaryotic-eukaryotic shuttle vector pSV2neo were constructed. Each of these plasmids was mixed with another pSV2neo plasmid containing a different, nonoverlapping deletion. Recombination was measured in mammalian cells and bacteria by the frequency of reconstruction of an intact neo gene. We observed that 25 base pairs of homologous sequence is sufficient to yield recombinant products, implying that synapsis and homologous pairing can occur with this level of homology. Examination of the products revealed that nonreciprocal recombination played a role in the generation of normal neo genes. In addition coconversion of linked markers was observed. Exonucleolytic action seems to play a role in gene conversion.