Fig. 6. DNA double-strand breaks might be responsible for the csn mutant growth arrest. (A) The IU.GUS transgene before and after DNA repair following a DNA double-strand break (DSB) in the interrupted GUS gene fragment. IU.GUS carries a GUS reporter gene, which is non-functional owing to an insertion in the GUS gene. In addition, this transgene construct carries a non-functional but uninterrupted 1087 bp GUS fragment, which corresponds to the GUS sequences that are upstream and downstream of the insertion in the first GUS gene. The rationale of the IU.GUS transgene is that a DSB within the interrupted GUS gene can be repaired by homologous recombination using the uninterrupted GUS fragment as template, thereby rendering the GUS gene active. LB, left border; RB, right border; P, promoter; T, terminator; Hygromycin, hygromycin resistance gene; GUS, intact GUS gene; GU^*-U^**S, interrupted GUS gene; U, uninterrupted GUS gene fragment. (B) Five-day-old light-grown wild-type and csn mutant Arabidopsis seedlings that contain IU.GUS following GUS-staining. GUS-positive cells (arrowheads) are detected in the csn mutants, but not in the wild type. Insets show GUS-positive cells at higher magnification. Scale bars: 1 mm in wild type; 0.5 mm in csn3, csn4 and csn5ab. (C) Quantitative analysis of the DNA repair events in wild-type and csn mutant seedlings. Both CSN5 genes reside on chromosome 1 and therefore the unlinked IU.GUS 7 (chromosome 5) was used for this cross. CSN3 and CSN4 are on chromosome 5, therefore IU.GUS 8 (chromosome 1) was used for this cross.