Isolation and characterization of cDNA clones for carrot extensin and a proline-rich 33-kDa protein

Extensins are hydroxyproline-rich glycoproteins associated with most dicotyledonous plant cell walls. To isolate cDNA clones encoding extensin, we started by isolating poly(A)(+) RNA from carrot root tissue, and then translating the RNA in vitro, in the presence of tritiated leucine or proline. A 33-kDa peptide was identified in the translation products as a putative extensin precursor because: (i) it is rich in proline and poor in leucine, and (ii) the message appears to be more abundant when carrot tissue is wounded. From a cDNA library constructed with poly(A)(+) RNA from wounded carrots, one cDNA clone (pDC5) was identified that specifically hybridized to poly(A)(+) RNA encoding this 33-kDa peptide. We isolated three cDNA clones (pDC11, pDC12, and pDC16) from another cDNA library using pDC5 as a probe. DNA sequence data, RNA hybridization analysis, and hybrid released in vitro translation indicate that the cDNA clone pDC11 encodes extensin and that cDNA clones pDC12 and pDC16 encode the 33-kDa peptide, which as yet has an unknown identity and function. The assumption that the 33-kDa peptide was an extensin precursor was invalid. RNA hybridization and DNA sequence analysis indicate that pDC5 is a hybrid clone corresponding to two RNA species. RNA hybridization analysis showed that RNA encoded by both clone types is accumulated upon wounding.