Reconstitution of floral quartets in vitro involving class B and class E floral homeotic proteins

Homeotic MADS box genes encoding transcription factors specify the identity of floral organs by interacting in a combinatorial way. The floral quartet model, published several years ago, pulled together several lines of evidence suggesting that floral homeotic proteins bind as tetramers to two separated DNA sequence elements termed CArG boxes by looping the intervening DNA. However, experimental support for floral quartet formation remains scarce. Recently, we have shown that the class E floral homeotic protein SEPALLATA3 (SEP3) is sufficient to loop DNA in floral-quartet-like complexes in vitro. Here, we demonstrate that the class B floral homeotic proteins APETALA3 (AP3) and PISTILLATA (PI) do only weakly, at best, form floral-quartet-like structures on their own. However, they can be incorporated into such complexes together with SEP3. The subdomain K3 of SEP3 is of critical importance for the DNA-bound heterotetramers to be formed and is capable to mediate floral quartet formation even in the sequence context of AP3 and PI. Evidence is presented suggesting that complexes composed of SEP3, AP3 and PI form preferentially over other possible complexes. Based on these findings we propose a mechanism of how target gene specificity might be achieved at the level of floral quartet stability.