shRNA Screen Modulators of p53 Figure 3. Barcode identified shRNA vectors suppress protein and mRNA levels of their targets. a) QRT-PCR for ARNTL in BJtsLT cells. BJtsLT cells were infected with indicated shRNA vectors. Samples for RNA isolation were taken 8 days after shift to 39uC. b) QRT-PCR for RBCK1 in BJtsLT cells. BJtsLT cells were infected with indicated shRNA vectors. Samples for RNA isolation were taken 8 days after shift to 39uC. c) QRT-PCR for TNIP1 in BJtsLT cells. BJtsLT cells were infected with indicated shRNA vectors. Samples for RNA isolation were taken 8 days after shift to 39uC. d) Flag- ARNTL together with the shRNA vectors targeting ARNTL were transiently transfected in Phoenix cells. Extracts were immunoblotted using Flag and CDK4 (control) antibodies. e) BJ cells were infected with the indicated shRNA vectors and Extracts were immunoblotted using TNIP1 and CDK4 (control) antibodies. doi:10.1371/journal.pone.0004798.g003 p21CIP1 continue proliferation after the forced expression of p19ARF (Figure 5b), knockdown of ARNTL also allows cells to proliferate after p53 activation by p19ARF (for knockdown see figure 5c). This result suggests that ARNTL expression is required for the anti- proliferative response of 19ARF activation. When ARNTL levels are low, the cells escape this arrest. PLoS ONE | www.plosone.org 6 March 2009 | Volume 4 | Issue 3 | e4798