ABSTRACT

Exchange of genetic material by recombination occurs in all living organisms and is the major contributor to high-quality diversity generation in the evolution of the species. The recombination apparatus is evolved to various degrees of complexity in various living organisms with a number of fine tunings and mechanisms that are still not completely understood. The homologous recombination frequency varies greatly between organisms (and between cell cycle stages including mitosis and meiosis). Saccharomyces cerevisiae has been used for many years as a eukaryotic model for studying homologous recombination (1). It has a high frequency of homologous recombination, e.g., as shown by the site-specific integration into the chromo-some upon transformation with DNA. On the contrary, Escherichia coli has a relatively low frequency of integration of homologous DNA into the chromosome, despite the fact that the E. coli chromosome is smaller than the S. cerevisiae chromosome. Using in vivo recombination in S. cerevisiae as a cloning tool of a fragment into a gapped plasmid with a 40 to 150 bp overlap at the ends has been described (2). This is a very efficient method for cloning a fragment in S. cerevisiae. A similar method in E. coli gives a relatively low recombination efficiency (3).