PRESENCE OF AN INTRON IN A GENE OF PAP-II, THE RIBOSOME-INACTIVATING PROTEIN FROM SUMMER LEAVES OF PHYTOLACCA-AMERICANA

High levels of a family of proteins called pokeweed antiviral protein (PAP) are found in various organs of Phytolacca americana. Polymerase chain reaction (PCR) amplification of genomic DNA from Phytolacca amer icana was used to clone and sequence the genomic genetic determinants of three members of the PAP family: PAP I, PAP II, and PAPS. The resul ts demonstrated that PAP I and PAP-S do not contain an intron, whereas one PAP II gene is composed of two exons separated by an intron of 73 4 basepairs in length. Since the amino acid sequence of PAP II is only 33% similar to PAP I and PAP-S, and since it seems unlikely that an i ntron has been inserted into a pre-existing gene, the PAP II gene coul d be the ancestor of the PAP family. Interestingly, PAP II expression is regulated differently in leaf and seed tissues and is the only PAP transcript which increases progressively with plant aging. The evoluti onary relationship of PAPs could help to identify relevant functional structures of these proteins and shed new light on structural function al models which attempt to explain their enzymatic action. (C) 1997 An nals of Botany Company.