(A) Scheme showing the main domains of ∆Np73-α and −β, as well as the truncations used in our experiments. At the right hand a western blot shows the expression of the different constructs. The same blot was stripped and re-incubated with an anti-Sp1 antibody. (B, C). Activation of the responsive element within BIK promoter by ∆Np73 lacking different important domains of the protein. Truncation of the C-terminus or the ODs greatly affects the transactivation capacity of ∆Np73-α (B). Elimination of the SAM domain or the 13 N-terminal isoform-distinctive residues has virtually no effect (B, C). RLA is represented. (D) Dominant negative effect of the OD of p73 over ∆Np73-α and Sp1 in the activation of the responsive element of BIK. An expression vector for the region between amino acids 290 and 337 of ∆Np73-α was transfected along with Sp1 or the full length ∆Np73-α to analyse their effect on the activation of the response element of BIK. RLA in the cell extracts is depicted. (E) Deletion of the N-terminal and DNA-binding domains of ∆Np73-α and −β abolish their transactivation capacity, but those truncated forms act as dominant negatives of the full length proteins. The indicated constructs were cotransfected and the RLA in the cell extracts determined. (F) p73DDα, embracing amino acids 327–636 of p73α [26], act as dominant negative of ∆Np73α, but the mutation L371P within its OD (p73DDαLP) impairs this effect. (G) ∆Np73-α proteins lacking the SAM or the C-terminal domains are still capable of synergizing with Sp1 in the activation of the responsive element in the BIK promoter. Contrarily, deletion of the OD of the protein completely abolish its transactivation and Sp1-synergizing capacities. RLA in the cells transfected with the indicated constructs is represented.