recombination and DNA break repair between distant sister loci that had segregated to opposite halves of the E. coli cell. E. coli strains deficient in RecA are useful for cloning procedures in molecular biology laboratories. E. coli strains are often genetically modified to contain a mutant recA allele and thereby ensure the stability of extrachromosomal segments of DNA, known as plasmids. In a process called transformation, plasmid DNA is taken up by the bacteria under a variety of conditions. Bacteria containing exogenous plasmids are called "transformants". Transformants retain the plasmid throughout cell divisions such that it can be recovered and used in other applications. Without functional RecA protein, the exogenous plasmid DNA is left unaltered by the bacteria. Purification of this plasmid from bacterial cultures can then allow high-fidelity PCR amplification of the original plasmid sequence. Potential as a drug target Wigle and Singleton at the University of North Carolina have shown that small molecules interfering with RecA function in the cell may be useful in the creation of new antibiotic drugs. Since many antibiotics lead to DNA damage, and all bacteria rely on RecA to fix this damage, inhibitors of RecA could be used to enhance the toxicity of antibiotics. Additionally the activities of RecA are synonymous with antibiotic resistance development, and inhibitors of RecA may also serve to delay or prevent the appearance of bacterial drug resistance. Role of RecA in natural transformation Based on analysis of the molecular properties of the RecA system, Cox concluded that the data "provide compelling evidence that the primary mission of RecA protein is DNA repair." In a further essay on the function of the RecA protein, Cox summarized data demonstrating that "RecA protein evolved as the central component of a recombinational DNA repair system, with the generation of genetic diversity as a sometimes useful byproduct." Natural bacterial transformation involves the transfer of DNA from one bacterium to another (ordinarily of the same species) and the integration of the donor DNA into the recipient chromosome by homologous recombination, a process mediated by the RecA protein (see Transformation (genetics)). Transformation, in which RecA plays a central role, depends on expression of numerous additional gene products (e.g. about 40 gene products in Bacillus subtilis) that specifically interact to carry out this process indicating that it is an evolved adaptation for DNA transfer. In B. subtilis the length of the transferred DNA can be as