The Tomato 66.3-kD Polyphenoloxidase Gene: Molecular Identification and Developmental Expression

Abstract

A gene coding for a polypeptide abundant in tomato floral meristems was isolated and shown to represent a tomato 66.3-kD polyphenoloxidase. Analysis of cDNA clones and a corresponding intronless genomic clone indicated that the plastid-bound 587-residue-long polypeptide, designated P2, contains two conserved copper-binding domains, similar to those found in funga1 and mammalian tyrosinases. P2 transcripts and polypeptides are accumulated in the arrested floral primordia of the anantha mutant inflorescences and are equally abundant in primordia of wild-type flowers; the gene continues to be expressed at high levels in developing floral organs. In young expanding leaves, P2 protein is concentrated in palisade cells and in epidermal trichomes. Expression patterns of P2 in plant meristems permit molecular distinction between floral and vegetative primordia, and, in a companion study, comparison with dUTPase suggests that the two genes mark two alternative complementary developmental programs in the floral and vegetative meristems of the tomato plants.

Import, Targeting, and Processing of a Plant Polyphenol Oxidase

“…This was suggested to be the precursor of the 59-kD purified trichomal PPO (Kowalski et al, 1992) and was further supported by the structure of cDNA clones isolated from potato (Hunt et al, 1993). Cloned tomato PPO genes appeared to code for 66.1-to 70.6-kD precursors of 57.0-to 61.6-kD mature products (Shahar et al, 1992;Yu, 1992;Newman et al, 1993). Gary et al (1992) reported recently that a full-length V. faba PPO cDNA clone had a predicted mature molecular mass of 58 kD.…”

“…The active form of PPO was frequently reported to be about 45 kD (Vaughan et al, 1975;Lanker et al, 1987), but other forms, e.g. 59 and 65 to 68 kD were also reported Ganesa et al, 1992;Shahar et al, 1992). PPOs appear to be encoded by gene families .…”

“…However, a11 plant PPO genes isolated so far encode about 67-kD polypeptides. Furthermore, a11 of these polypeptides possess amino-terminal extensions with properties typical of nuclear-coded plastid proteins Cary et al, 1992;Shahar et al, 1992;Hunt et al, 1993;Newman et al, 1993). In vitro translation of potato trichome poly(A+) RNA produces a 67-kD polypeptide, whereas the native enzyme is 59 kD Kowalski et al, 1992;Hunt et al, 1993).…”

The 60‐kDa precursor to the dithiothreitol‐sensitive tetrameric protease of spinach thylakoids: structural similarities between the protease and polyphenol oxidase

“…However, it was suggested recently that the sequence could have been derived from polyphenol oxidase (PPO) that had not been separated from the kinase on SDS-PAGE (I. Ohad, unpublished). The mature PPOs from other plants have similar N-terminal sequences [10][11][12][13]. Thus, the 60-kDa protein is likely to be identical to PPO.…”

“…Cloning of cDNAs and genes yielded deduced amino acid sequences that indicated that the protein is translated as a precursor with a transit sequence that would direct the precursor into the thylakoid lumen [10][11][12][13]. Nevertheless, the physiological function of this protein remains to be established [14] because there is no reason to believe that o-diphenol substrates of PPO would be present in the thylakoid lumen.…”