MAIZE MITOCHONDRIAL SERYL-TRANSFER-RNA SYNTHETASE RECOGNIZES ESCHERICHIA-COLI TRNA(SER) IN-VIVO AND IN-VITRO

In our studies to analyze the structure/function relationships among c ytoplasmic and organellar seryl-tRNA synthetases (SerRS), we have char acterized a Zea mays cDNA (SerZMm) encoding a protein with significant similarity to prokaryotic SerRS enzymes. To demonstrate the functiona l identity of SerZMm, the gene sequence encoding the putative mature p rotein was cloned. This construct complemented in vivo a temperature-s ensitive Escherichia coil serS mutant strain. The mature SerZMm protei n overexpressed in Escherichia coli efficiently aminoacylated bacteria l tRNA(Ser) in vitro, while yeast tRNA was a poor substrate. These dat a identify SerZMm as an organellar maize seryl-tRNA synthetase, the fi rst plant organellar SerRS to be cloned. The analysis of its N-termina l targeting signal suggests a mitochondrial function for the SerZMm pr otein in maize.