Characterization of a flower-specific gene encoding a putative myrosinase binding protein in Arabidopsis thaliana

A cDNA clone, 4B-1, previously isolated by differential screening is prefer entially expressed in floral organs of Arabidopsis thaliana. Characterizati on of the full length cDNA and the genetic locus corresponding to 4B-1 cDNA revealed that it potentially encodes a myrosinase binding protein (MBP) wh ich is presumably present in a large myrosinase complex. The deduced amino acid sequence of the polypeptide encoded by cDNA clone (designated f-AtMBP) appeared to consist of two parts: one region at the C-terminal half repres enting overall homology with AtMBP, an MBP homologue in A. thaliana, and th e other at an extended N-terminal region of about 150 amino acids showing s ignificant identity with the N-terminal region of the MBP-related protein r eported in Brassica. Expression analysis by RNA blot and in situ hybridizat ion showed that f-AtMBP was specifically expressed in floral meristems, pis tils, stamens, petals, and ovules of immature flowers, but no expression wa s observed in the specialized cells called the myrosin cells in the hypocot yl and cotyledons of developing seeds where myrosinase enzymes are normally found. Although MBPs and MBP-related proteins are considered to be inducib le by exogenous application of signal molecules and physical wounding, we f ound that f-AtMBP expression was not activated by such treatment, suggestin g that f-AtMBP is a novel type of MBP specific to floral organs.