CLONING AND DEVELOPMENTAL EXPRESSION OF PEA RIBULOSE-1,5-BISPHOSPHATECARBOXYLASE OXYGENASE LARGE SUBUNIT N-METHYLTRANSFERASE

Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) large subuni t (LS) N-methyltransferase (Protein methylase III, Rubisco LSMT, EC 2. 1.1.43) catalyzes methylation of the E-amino group of Lys-14 in the LS of Rubisco. With limited internal amino acid sequence information obt ained from HPLC-purified peptic polypeptides from Rubisco LSMT, a full -length cDNA clone was isolated utilizing polymerase chain reaction-ba sed technology and conventional bacteriophage library screening. The 1 802 bp cDNA of Rubisco LSMT encodes a 489 amino acid polypeptide with a predicted molecular mass of ca. 55 kDa. A derived N-terminal amino a cid sequence with features common to chloroplast transit peptides was identified. The deduced sequence of Rubisco LSMT did not exhibit regio ns of significant homology with other protein methyltransferases. Sout hern blot analysis of pea genomic DNA indicated a low gene copy number of Rubisco LSMT in pea. Northern analysis revealed a single mRNA spec ies of about 1.8 kb encoding for Rubisco LSMT which was predominately located in leaf tissue. Illumination of etiolated pea seedlings showed that the accumulation of Rubisco LSMT mRNA is light-dependent. Maximu m accumulation of Rubisco LSMT transcripts occurred during the initial phase of light-induced leaf development which preceded the maximum ac cumulation of rbcS and rbcL mRNA. Transcript levels of Rubisco LSMT in mature light-grown tissue were similar to transcript levels in etiola ted tissues indicating that the light-dependent accumulation of Rubisc o LSMT mRNA is transient. This is the first reported DNA and amino aci d sequence for a protein methylase III enzyme.