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 preferentially expressed in floral organs of Arabidopsis thaliana. Characterization of the full length cDNA and the genetic locus corresponding to 4B-1 cDNA revealed that it potentially encodes a myrosinase binding protein (MBP) which 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 representing overall homology with AtMBP, an MBP homologue in A. thaliana, and the other at an extended N-terminal region of about 150 amino acids showing significant identity with the N-terminal region of the MBP-related protein reported in Brassica. Expression analysis by RNA blot and in situ hybridization showed that f-AtMBP was specifically expressed in floral meristems, pistils, stamens, petals, and ovules of immature flowers, but no expression was observed in the specialized cells called the myrosin cells in the hypocotyl and cotyledons of developing seeds where myrosinase enzymes are normally found. Although MBPs and MBP-related proteins are considered to be inducible by exogenous application of signal molecules and physical wounding, we found that f-AtMBP expression was not activated by such treatment, suggesting that f-AtMBP is a novel type of MBP specific to floral organs.