Chi: a little sequence controls a big enzyme.

WHEN we stumbled over Chi in coliphage (in “Benzerize” the region of the chromosome that contained the known recombination genes (Figure 2). [Sey1972?), it appeared to be a uniquely accessible example of a “recombination initiator,” whose existence mour Benzer (1961) had created a set of overlapping deletions within the rII region of phage T4 as a device for was implied by gene-conversion gradients (polarons) of fungi. Hence, it promised to have wide significance for rapidly mapping any newly arising rII mutation.] Among ’s recombination genes are gam, whose product inactiour understanding of meiotic as well as of prokaryotic recombination. For a time, Chi seemed to fulfill its vates E. coli’s recombination-related nuclease RecBCD, and red, which supplies recombination enzymes more promise, but things turned out otherwise. Nevertheless, Chi did elucidate basic aspects of genetic recombination compatible with ’s life style. Among the deletions that David sought were those that were missing both red and genome maintenance, played a role in the development of as a cloning vehicle, and continues to bring and part of gam. Phage missing red and gam had been identified previously as pbio-transducing phage. These enzymological surprises. In this Perspectives, which reflects my rather personal pbio phage occasionally arise in lysogenic cells when prophage, excising itself carelessly from the E. coli memory of events, literature citations of work from our lab are omitted to improve readability. They can be chromosome, picks up the E. coli bio gene in place of its own recombination region (see Anderson 1987). found in older reviews (e.g., Myers and Stahl 1994; Smith 1998) or electronically. For those in a hurry, Such phage can be selected for by their novel ability to make (pretty-good-sized) plaques on a P2 lysogen of E. here is the bottom line: Escherichia coli’s RecBCD enzyme enters duplex DNA at a double-strand break and travels coli (the “Spi ” phenotype), a property shown to depend on the loss of both the red and gam gene functions. To in a destructive mode until it encounters a properly select Spi phage that were pure deletions, instead of oriented octamer called Chi. This encounter civilizes bio substitutions, David exploited the observation that the enzyme, which keeps on traveling, in a recombinaparticles with chromosomes shortened by deletions are genic mode, recruiting E. coli’s strand-invasion protein, relatively resistant to the destabilizing effects of the RecA, to effect recombination when a homolog is availMg -chelating agent, EDTA. Using a phage stock that able. The primary adaptive significance of Chi is likely had been grown in the lytic cycle, rather than having to concern E. coli DNA replication, when breaks occur been induced from the prophage state, also helped to at the fork. Since these breaks are repaired by a RecBCDavoid bio substitutions. David did obtain a set of red gam promoted recombination-like reaction (usually between deletions but found that they all made tiny (“pin-prick”) the two tines of the fork), Chi plays a role in the mainteplaques. Initial attempts to grow them to a useful titer nance of the E. coli genome. failed. Undaunted, David kept trying to grow them and Discovery: In 1969–1970, Mary Morgan Stahl (Figure succeeded when his Darwinian exercise resulted in de1) and I enjoyed a sabbatical leave with Noreen and Ken rivatives with pretty good plaque size. He showed that Murray in Edinburgh, where we pursued our studies each of these variants was the result of one or another on relationships between DNA replication and genetic single mutation, usually distant from the red gam region, recombination in the coliphage . During this period, that suppressed specifically the poor growth of the red Noreen’s undergraduate honors student, David Hendergam deletion mutants. (These “Henderson suppressors” son, encountered a curious phenomenon while trying to were the as-yet-unnamed Chi mutations.) David’s largeplaque variants still retained the originally selected deletion and were red gam mutants, as indicated by their 1Author e-mail: [email protected]