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The present investigation was undertaken to obtain molecular data of a new immunoglobulin (Ig)E-binding birch pollen protein with a mass of 35 kDa. In a previous study, this protein showed IgE cross-reactivity with 34- and 35-kDa proteins in apples, pears, carrots, bananas and other exotic fruits. Since the protein was N-terminally blocked, it was purified by preparative SDS-PAGE, and multiple proteolytic fragments were subsequently generated by in-gel digestion with the endoproteinases Glu C, Lys C and Clostripain. After electrophoretic separation and blotting onto polyvinylidene difluoride(PVDF), the resulting polypeptides were subjected to N-terminal amino acid microsequencing. The internal sequences obtained showed a high degree of sequence identity to isoflavone reductases (IFR) and isoflavone reductase-like proteins (IRL) from several plants which also had a similar size. For a stretch of 25 consecutive residues this identity ranged from 56% for IFR from peas and chick peas and an IRL from maize, to 80% for a tobacco IRL. A 453 bp fragment was amplified from total birch pollen RNA by polymerase chain reaction (PCR) using primers derived from the nucleotide sequence of the tobacco IRL. The deduced 151 amino acid sequence represented ≈ 50% of the protein and confirmed the sequence identities obtained by Edman degradation. Moreover, the 25 amino acid sequence was included in the cloned fragment. Deduced and determined amino acids showed only one mismatch, which was due to a single nucleotide exchange. At the antibody level, the immunological relationship of the birch pollen protein to IRL and IFR was demonstrated by immunoblotting with a rabbit antiserum against a pea IFR which recognized the same birch protein as patients' IgE. The rabbit antiserum also reproduced the cross-reactivity pattern previously observed with patients' IgE by recognizing related proteins in specific plant foods, including some exotic fruits. We therefore suggest that the 35-kDa birch pollen protein belongs to the IFR/IRL family and represents a minor allergen, possibly being responsible for less common pollen-related food allergies in patients allergic to birch pollen.