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The following HTML text is provided to enhance online readability. Many aspects of typography translate only awkwardly to HTML. Please use the page image as the authoritative form to ensure accuracy. radiation, whereas those in S phase responded efficiently. A single fraction (no. 7) contained predominantly G2 cells and this fraction showed less focus induction than did the previous fraction that contained mostly S-phase cells at the time of radiation treatment. However, this result could be misleading because the number of cells recovered in fraction no. 7 was quite low, reducing the reliability of FACS analysis. Furthermore, in other experiments, G2-enriched fractions showed efficient induction of Rad51 foci (data not shown). In contrast to the variable result with G2-enriched fractions, we consistently see failure of G1 cells to induce Rad51 foci and efficient induction of foci in S-phase cells. Induction of recombinational repair capability in S phase makes biological sense, in that most of the spontaneous DNA damage repaired by the recombinational mechanism occurs as a result of detects in replication (see ref. 103 and references therein). Although changes in Rad51 expression level alone could account for the failure of G1 (and possibly G2) cells to form Rad51 foci in response to radiation, other regulatory processes may influence this response as well. Several lines of evidence from mammalian cells and yeast indicate that RPA, Rad51, and Rad55 are all phosphorylated in response to radiation (104– 108). Phosophorylation of these proteins is mediated by members of the ATM family of protein kinases in mammals and yeast. The activity of these kinases is induced in response to damage. ATM-dependent phosphorylation of Rad51 is mediated indirectly through ATM-dependent activation of the nonreceptor tyrosine kinase c-Abl (107, 108). One study found that phosphorylation of Rad51 by c-Abl increased Rad51-Rad52 interaction (108), a change that would be expected to enhance assembly of Rad51 in response to damage. However, another study found that phosphorylated Rad51 had reduced ability to bind ssDNA and was inactive in strand exchange reactions (107). Perhaps two pools of Rad51 are required for an efficient repair after damage, a phosphorylated pool to promote filament initiation via Rad52 interaction and an unphosphorylated pool critical for filament elongation and strand exchange. Another possibility is that phosphorylation of Rad51 contributes to cell-cycle regulation of Rad51 activity. Further studies are required to determine the role of posttranslational modification in assembly of recombination complexes in vivo and in vitro. Future Directions for the Study of Recombinase Assembly. Substantial progress has been made in the last few years on the mechanisms that promote assembly of RecA-like recombinases. Important questions remain to be addressed. What are the structural intermediates involved in recombinase assembly? How is mediator activity regulated to ensure that recombinase assembles only at sites of DNA damage and not on the tracts of ssDNA that form as normal intermediates in DNA replication? Does mediator binding at DNA ends play an important part in recombinase assembly? Are the requirements for assembly at ends different from the requirements for assembly at daughter strand gaps? Why are mediator requirements stricter in meiosis than in mitosis? Does down-regulation of Rad51 protein alone account for the inability of G1 cells to form Rad51 foci in response to damage, or are other regulatory mechanisms in place? What role does damage-dependent phosphorylation play in recombinase assembly? Bacteriophages have often provided paradigms of broad relevance to biological systems. This is certainly true in the case of recombinase assembly where the lessons learned from phage have guided experiments in bacteria, yeast, and higher eukaryotes. On the other hand, demonstration of different mediator requirements in mitosis and meiosis contributes to evidence indicating that some of the mechanisms regulating assembly of recombination complexes are unique to eukaryotes. We thank Steve Kowalczykowski, Tim Lohman, Shunichi Takeda, Scott Morrical, John Petrini, and Akira Shinohara for helpful conversations. We also thank Jeff Murley and David Grdina for technical advice on centrifugal elutriation. This work was supported by a National Institute of General Medical Sciences Grant GM50936 awarded to D.K.B. 1. Howard, F.P., West, S.C. & Stasiak, A. (1984) Nature (London) 309, 215–219. 2. Stasiak, A. & Egelman, E.H. (1994) Experientia 50, 192–203. 3. Griffith, J. & Formosa, T. (1985) J. Biol. Chem. 260, 4484–4491. 4. Sung, P. & Robberson, D.L. (1995) Cell 82, 453–461. 5. 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