Seryl-tRNA synthesis in maize

Background and purpose: The translation of a nucleotide sequence into an amino acid sequence requires a full complement of aminoacylated tRNAs. The accuracy of aminoacyl-tRNA synthesis rests on the specificity of the aminoacyl-tRNA synthetases which catalyze aminoacylation of tRNAs by joining an amino acid to its cognate tRNA. In higher plants, protein synthesis takes place in three cellular compartments: cytosol, mitochondria and chloroplasts. These compartments generally require their own sets of tRNAs and aminoacyl-tRNA synthetases. To investigate the enzymes involved in serylation, we cloned cDNAs encoding cytosolic and organellar seryl-tRNA synthetases. Materials and methods:, cDNA clones identified in a search of the Zea mays expressed sequence tag (EST) data base which encodes polypeptides with significant similarity to the published seryl-tRNA synthetase (SerRS) sequences, were taken for complete sequencing and further analyses. Results: There are at least two dissimilar genes for SerRS present in the maize genome. The clones with the longest cDNA inserts, designated SerZMm and SerZMc, encode the open reading frames of 489 and 451 amino acids, respectively The deduced amino acid sequence of the SerZMm translation product was found to be more similar to mitochondrial and bacterial SerRS enzymes, while the SerZMc shared significant homology with eukaryotic cytoplasmic counterparts. Conclusion: All the characteristic structural motifs found in seryl-tRNA synthetases from various sources are conserved in maize SerRS enzymes. The SerZMm has a SO-amino acid sequence at the N-terminus that resembles a transit peptide for localization to mitochondria, while SerZMc comprises a basic C-terminal extension typical for eukaryotic seryl-tRNA synthetases.