Figure 6

Proposed model for effect of the R69L substitution on interaction specificity with protein partners and consequently on trans-activation properties of VvMYB5b in tobacco flowers. X, Y and Z indicate endogenous transcription factors expressed in corolla and/or stamens of tobacco flowers. MYB is a tobacco endogenous transcription factor normally expressed in petals and involved in anthocyanin synthesis in cooperation with endogenous partner, such as a bHLH protein. In transgenic petals, both VvMYB5b (mutated or normal) are able to recognize endogenous partners and to activate promoters of CHS and ANS encoding genes. In the particular case of NtDFR promoter, our results suggest the R69L mutation may change the DNA binding specificity of the protein complex, because VvMYB5b^L activated NtDFR transcription, contrary to VvMYB5^R. In wild-type stamens, anthocyanin biosynthetic pathway is not active, but transcription factors (Y) involved in other processes should be present. In transgenic stamens, VvMYB5b^R may be able to recognize this(ese) partner(s) to activate promoters, while VvMYB5b^L may not. Putative WDR factors, which have been shown in numerous models to be part of the complex, are not indicated in the figure.