The cDNA sequence of two MADS box proteins in Petunia.

Severa1 homeotic genes that are involved in floral differentiation have recently been isolated and characterized from Antirrhinum majus and Arabidopsis thaliana (Sommer et al., 1990; Yanofsky et al., 1990). Based on genetic and molecular studies, a model has been proposed to explain the roles of different homeotic genes in the specification of floral organ identity (Bowman et al., 1991). To see whether this model might also apply to Petunia, we initiated a project to isolate Petunia genes that show sequence homology to known floral homeotic genes in Antirrhinum and Arabidopsis. A11 of the floral homeotic genes isolated to date contain a putative DNA-binding domain that shows homology with that of other so-called MADS box proteins (Schwarz-Sommer et al., 1990; Angenent et al., 1992). We used a polymerase chain reaction technique to synthesize a DNA fragment encoding the MADS box region (amino acids 2-93) of Antirrhinum DEF A (Sommer et'al., 1990). This 280-bp polymerase chain reaction fragment was used as a probe to screen a Petunia hybrida W115 petal cDNA library. Twelve independent clones were purified and excised from X Zap I1 according to the manufacturer's instructions (Stratagene). Nine clones were found to differ in their restriction pattems. The nucleotide sequences of two clones (Petunia MADS 1 and 2, Tables I and 11) were detennined using the United States Biochemical Sequenase kit. The nucleotide sequence of the PMADS 1 cDNA is 881 bp in length and encodes a protein with 231 amino acid residues and a predicted molecular mass of 27,022 D. Sequence comparison shows that PMADS 1 is highly homologous to the DEF A gene of Antirrhinum majus (Sommer et al., 1990). The amino acid homology is 93% within the MADS box region (amino acids 3-60) and 77% in the remainder of the protein. Northem blot analysis showed that PMADS 1 is preferentially expressed in the second and third whorl of Petunia flowers (van der Krol et al., 1993). No expression of PMADS 1 was observed in leaves. Consistent with this expression pattern, we found that the PMADS 1 gene is deleted from a green petal mutant that shows a homeotic conversion of petal