p53 Response to Senescence Figure 1. Experimental system. A) Molecular markers used to identify senescence. B) Scheme of the procedure (see M & M) and 2 representative images of the dot blot obtained after hybridization. C) Comparison of several conditions of well-known activated targets of p53. Western blot showed no variation among p53 levels under comparable conditions. doi:10.1371/journal.pone.0003230.g001 and (3) cells showing basal levels of p53 activity. There was also one condition of replicative senescence with a moderate increase of p53 activity (P5). Oncogenic Ras activation seems to induce higher levels of p53 activation with similar senescent phenotypes (P3+ras). However, elevated levels of p53 do not always induce senescence as p53ts cells at permissive temperature are reversibly arrested, but p53 activity is higher than in the two previous conditions displaying senescence. As before, oncogenic Ras expression switches the cell from arrest to senescence, also increasing the relative p53-dependent transcription (Table 1 and Figure 2A). PLoS ONE | www.plosone.org 3 September 2008 | Volume 3 | Issue 9 | e3230