LIG1 and LIG3 contribute to the survival of cells exposed to IR.

<p>(<b>A</b>) Cell survival measured by colony formation in wt, <i>LIG1^−/−</i>, <i>LIG4</i>^−/− and <i>LIG1^−/−LIG4^−/−</i> cells after exposure to increasing doses of X-rays. Results from three independent experiments with 3 replicates each were used to calculate the indicated means and standard errors. (<b>B</b>) As in A. for <i>LIG3^−/2loxP</i>, and <i>LIG3^−/−Cdc9</i> cells after treatment for 1 h before and 4 h after IR with 10 µM NU7441. The dashed lines trace for comparison the results of wt and <i>LIG1^−/−LIG4</i>^−/− cells. (<b>C</b>) As in B. for <i>LIG3^−/2loxP</i>, <i>LIG3^−/M2I, LIG3^−/2loxPLIG4^−/−</i>, and <i>LIG3^−/M2ILIG4</i>^−/− cells. Results from at least two independent experiments with 3 replicates each were used to calculate the indicated means and standard errors. (<b>D</b>) As in B. for clone 3 of <i>LIG3^−/2loxPLIG4^−/−mts-hLIG1</i> and <i>LIG3^−/−LIG4^−/−mts-hLIG1.</i> Results from at least two independent experiments with 3 replicates each were used to calculate the indicated means and standard errors.</p

Dominant contribution of LIG3 <i>in-vitro</i> end joining.

<p>(<b>A</b>) Representative gels of <i>in vitro</i> DNA end joining of <i>Sal</i> I linearized <i>pSP65</i> plasmid using whole cell extracts of asynchronous wt, <i>LIG1^−/−</i>, <i>LIG4^−/−</i>, and <i>LIG1^−/−LIG4^−/−</i> cells. The linearized input substrate (linear) and the rejoined products (circles, dimers and multimers) generated by end joining are indicated (<b>B</b>) As in A. for <i>LIG3^−/2loxP</i> cells after treatment with 4HT for the indicated periods of time. (<b>C</b>) As in A. for <i>LIG3^−/2loxPLIG4</i>^−/− cells after treatment with 4HT for the indicated periods of time.</p

LIG3 processes IR-induced DSBs in LIG1 and LIG4 deficient cells.

<p>(<b>A</b>) Representative dose response curves for the induction of DSBs, as measured by PFGE, in cells exposed to increasing doses of X-rays. Images of ethidium bromide stained gels (upper panel) were analyzed to estimate the fraction of DNA released (FDR) from the well into the lane (regions defined as indicated) that is plotted as a function of IR dose for the indicated mutants (lower panel). Results from three independent experiments with 3 samples each were used to calculate the indicated means and standard errors. The dotted lines indicate the approach used to deduce Deq from FDR in DSB repair experiments (see text for details). (<b>B</b>) Repair kinetics of IR-induced DSBs in asynchronous wt, <i>LIG1^−/−</i>, <i>LIG4^−/−</i>, and <i>LIG1^−/−LIG4^−/−</i> cells after exposure to 40 Gy X-rays. The upper panel shows typical gels used to calculate the FDR at each repair time point, which was subsequently converted to Deq with the help of dose response curves such as those shown in A but generated with the same cell population used in the repair experiment (see text for details). Results of three determinations from at least two independent experiments were used to calculate the indicated means and standard errors (lower panel).</p

LIG3 supports processing of DSBs also after low doses of radiation.

<p>(<b>A</b>) Representative images of γ-H2AX foci formation in wt, <i>LIG1^−/−</i>, <i>LIG4^−/−</i> and <i>LIG1^−/−LIG4^−/−</i> DT40 cells after exposure to 1 Gy X-rays at the indicated times after IR. (<b>B</b>) Representative kinetics of γ-H2A.X foci formation and decay of wt DT40 cells as measured by immunostaining after exposure to 0.5 and 1 Gy X-rays. The results shown represent the analysis of 4000 nuclei in one representative experiment. (<b>C</b>) γ-H2A.X foci scored in wt, <i>LIG1^−/−</i>, <i>LIG4^−/−</i> and <i>LIG1^−/−LIG4^−/−</i> cells 8 h after exposure to 1 Gy X-rays. Foci measured in non-irradiated cells have been subtracted. Results of two independent experiments, in which 8000 nuclei were scored, were used to calculate the indicated means and standard errors.</p

Conditional knockout of LIG3 reveals the function of LIG1 in the processing of IR-induced DSBs.

<p>(<b>A</b>) Kinetics of DSB processing in the indicated mutants after treatment with 4HT for the indicated periods of time. Other details are as in Fig. 1B. Results from three independent experiments with 3 samples each were used to calculate the indicated means and standard errors. (<b>B</b>) <i>LIG3</i> mRNA levels measured by real-time PCR in wt and <i>LIG3^−/2loxP</i> cells after different incubation times with 4HT. The mRNA level measured in wt cells was set to 100%. (<b>C</b>) Western blot analysis of LIG3 protein in <i>LIG3^−/2loxP</i> cells after treatment with 4HT for the indicated periods of time. A mouse monoclonal antibody against human LIG3 (clone 1F3) that recognizes the chicken LIG3 was used. GAPDH is a loading control. (<b>D</b>) Apoptotic index measured by microscopically scoring nuclear fragmentation and pycnosis in wt and <i>LIG3^−/2loxP</i> cells at various times after treatment with 4HT. Results from two independent experiments in each of which 1000 cells were scored were used to calculate the indicated means and standard errors. (<b>E</b>) As in A for the indicated mutants. (<b>F</b>) Representative cell-cycle distribution histograms obtained by flow cytometry in wt, <i>LIG4^−/−</i> and <i>LIG3^−/2loxPLIG4^−/−</i> cells treated with 4HT for 3.5 days before and after enrichment by centrifugal elutriation in G2 phase of the cell cycle. (<b>G</b>) Kinetics of DSB processing in cells enriched by centrifugal elutriation in the G2 phase of the cell cycle as shown in F.</p