Grasses Like Mammals? Redundancy and Compensatory Regulation within the Retinoblastoma Protein Family

The retinoblastoma (RB) protein family plays a conserved and inhibitory role incell cycle progression in higher eukaryotes. In mammals, this family includes, in additionto RB, the related (RBR) proteins p107 and p130, which appear to have both specific andredundant functions compared to those of the prototypical RB protein. Whereas mostplant species seem to possess only one RBR gene, a recent study has shown that in maizethere are two types of distinctly regulated RBR proteins, RBR1 and RBR3. Expression ofRBR3 RNA is controlled by the RBR1-E2F pathway, and it is up-regulated uponinhibition of RBR1 activity by the wheat dwarf virus RepA protein in tissue culture,indicating the presence of a specific compensatory mechanism sustaining high pocketprotein activity. Database mining and phylogenetic analyses suggest the presence of twodistinct RBR genes to be a unique feature of grasses among plants, which might help toexplain their recalcitrance to genetic transformation.