Structure and Characterization of a cDNA Clone for Phenylalanine Ammonia-Lyase from Cut-Injured Roots of Sweet Potato

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Abstract
A cDNA clone for phenylalanine ammonia-lyase (PAL) induced in wounded sweet potato (Ipomoea batatas Lam.) root was obtained by immunoscreening a cDNA library. The protein produced in Escherichia coli cells containing the plasmid pPAL02 was indistinguishable from sweet potato PAL as judged by Ouchterlony double diffusion assays. The Mr of its subunit was 77,000. The cells converted [14C]-l-phenylalanine into [14C]-t-cinnamic acid and PAL activity was detected in the homogenate of the cells. The activity was dependent on the presence of the pPAL02 plasmid DNA. The nucleotide sequence of the cDNA contained a 2121-base pair (bp) open-reading frame capable of coding for a polypeptide with 707 amino acids (Mr 77, 137), a 22-bp 5′-noncoding region and a 207-bp 3′-noncoding region. The results suggest that the insert DNA fully encoded the amino acid sequence for sweet potato PAL that is induced by wounding. Comparison of the deduced amino acid sequence with that of a PAL cDNA fragment from Phaseolus vulgaris revealed 78.9% homology. The sequence from amino acid residues 258 to 494 was highly conserved, showing 90.7% homology.