Lim15/Dmc1 enhances DNA topoisomerase II catenation activity independent of sequence homology.

We reported previously that Coprinus cinereus Lim15/Dmc1 (CcLim15), a meiosis-specific recA-like protein, could specifically activate C. cinereus DNA topoisomerase II (CcTopII). In particular, it enhanced the catenation activity of CcTopII in vitro at the meiotic prophase stage (Iwabata K, Koshiyama A, Yamaguchi T, Sugawara H, Hamada NF, Namekawa HS, Ishii S, Ishizaki T, Chiku H, Nara T, Sakaguchi K, Nucleic Acids Res, 33:5809-5818, 2005). In this study, the interaction between CcTopII and CcLim15, especially during catenation, was investigated in detail using atomic force microscopy. We demonstrated earlier that CcLim15 enhanced the catenation activity of CcTopII in a dose-dependent manner. When using two different-sized plasmid rings (5.4 and 3 kbp), which did not have any homologous sequence regions, equal proportions of homologous and heterologous catenanes were produced, suggesting that CcLim15 causes an increase in catenation activity irrespective of the presence of homologous sequences between the rings. We also showed that CcLim15 works as a DNA-condensing agent. Therefore, we speculate that CcLim15 may work as a DNA-condensing factor specific to the zygotene event and that CcTopII is likely to resolve tangles when the chromosomes initiate pairing at multiple sites by CcLim15.