| Popis: |
WHEN we stumbled over Chi in coliphage λ (in 1972?), it appeared to be a uniquely accessible example of a “recombination initiator,” whose existence was implied by gene-conversion gradients (polarons) of fungi. Hence, it promised to have wide significance for our understanding of meiotic as well as of prokaryotic recombination. For a time, Chi seemed to fulfill its promise, but things turned out otherwise. Nevertheless, Chi did elucidate basic aspects of genetic recombination and genome maintenance, played a role in the development of λ as a cloning vehicle, and continues to bring enzymological surprises. In this Perspectives, which reflects my rather personal memory of events, literature citations of work from our lab are omitted to improve readability. They can be found in older reviews (e.g., Myers and Stahl 1994; Smith 1998) or electronically. For those in a hurry, here is the bottom line: Escherichia coli's RecBCD enzyme enters duplex DNA at a double-strand break and travels in a destructive mode until it encounters a properly oriented octamer called Chi. This encounter civilizes the enzyme, which keeps on traveling, in a recombinagenic mode, recruiting E. coli's strand-invasion protein, RecA, to effect recombination when a homolog is available. The primary adaptive significance of Chi is likely to concern E. coli DNA replication, when breaks occur at the fork. Since these breaks are repaired by a RecBCD-promoted recombination-like reaction (usually between the two tines of the fork), Chi plays a role in the maintenance of the E. coli genome. |
