Cdc25C (cell division cycle 25C) phosphatase triggers entry into mitosis in the cell cycle by dephosphorylating cyclin B-Cdk1. Cdc25C exhibits basal phosphatase activity during interphase and then becomes activated at the G 2 /M transition after hyperphosphorylation on multiple sites and dissociation from 14-3-3. Although the role of Cdc25C in mitosis has been extensively studied, its function in interphase remains elusive. Here, we show that during interphase Cdc25C suppresses apoptosis signal-regulating kinase 1 (ASK1), a member of mitogen-activated protein (MAP) kinase kinase kinase family that mediates apoptosis. Cdc25C phosphatase dephosphorylates phospho-Thr-838 in the activation loop of ASK1 in vitro and in interphase cells. In addition, knockdown of Cdc25C increases the activity of ASK1 and ASK1 downstream targets in interphase cells, and overexpression of Cdc25C inhibits ASK1-mediated apoptosis, suggesting that Cdc25C binds to and negatively regulates ASK1. Furthermore, we showed that ASK1 kinase activity correlated with Cdc25C activation during mitotic arrest and enhanced ASK1 activity in the presence of activated Cdc25C resulted from the weak association between ASK1 and Cdc25C. In cells synchronized in mitosis following nocodazole treatment, phosphorylation of Thr-838 in the activation loop of ASK1 increased. Compared with hypophosphorylated Cdc25C, which exhibited basal phosphatase activity in interphase, hyperphosphorylated Cdc25C exhibited enhanced phosphatase activity during mitotic arrest, but had significantly reduced affinity to ASK1, suggesting that enhanced ASK1 activity in mitosis was due to reduced binding of hyperphosphorylated Cdc25C to ASK1. These findings suggest that Cdc25C negatively regulates proapoptotic ASK1 in a cell cycle-dependent manner and may play a role in G 2 /M checkpointmediated apoptosis.

Polychlorinated biphenyl dechlorinating microbial populations in St. Lawrence River sediments were fractionated and estimated based on the dechlorination pattern using a combination of serial dilution and most probable number techniques. Two distinctive dechlorination patterns were found in most probable number sediments spiked with Aroclor 1248. A high-dilution inoculum decreased the average number of chlorines per biphenyl from 4.0 to 3.4 but was unable to dechlorinate meta-substituted congeners consisting mainly of 2,5,2',5'-, 2,4,2',5'-, and 2,5,2'-chlorobiphenyl (pattern B). On the other hand, a low-dilution inoculum did dechlorinate the meta-rich congeners and reduced the average number of chlorines to 2.9 (pattern A). These results indicate that there are at least two populations. While pattern B was produced by pattern B producing dechlorinators, pattern A was produced by a combination of pattern B producers plus another population that dechlorinates the meta-substituted congeners. When the population size was calculated based on the frequency of respective dechlorination patterns, the populations yielding pattern B were approximately 2.4 × 106cells·g dry weight sediment-1whereas the dechlorinators of the meta-rich congeners were two orders of magnitude less at 3.5 × 104cells·g dry weight sediment-1. Despite lower numbers, these meta-dechlorinators in pattern A increased the overall dechlorination by almost twofold.