Structures and expression patterns of two cDNA clones encoding S-adenosyl-L-methionine synthetase from the root nodule of Elaeagnus umbellata

Abstract

Two cDNA clones encoding S-adenosyl-L-methionine synthetase (SAMS) were isolated from the root nodule cDNA library of Elaeagnus umbellata Thunberg and analysed on the basis of deduced amino acid sequence and expression pattern. Two EuSAMS clones shared 75–84% identity at the nucleotide level, and 85–95% identity at the amino acid level, with the other plant SAMS genes. Genomic Southern hybridization revealed the presence of more than two copies of SAMS genes in the genome of E. umbellata. Reverse transcriptase-mediated polymerase chain reaction (RT–PCR) analysis showed EuSAMS1 transcripts were more abundant than those of EuSAMS2. Similar to the expression pattern of other plant SAMS genes, both genes were expressed at higher levels in root than in leaf. During nodule development, expression of both genes was increased, with the highest level at 6–8 week after inoculation, and decreased rapidly thereafter. In situ hybridization analysis also showed both SAMS transcripts in the meristem zone, the infected cells of the fixation zone and in the central vascular system of root nodules. However, EuSAMS2 transcripts were strongly detected in the prefixation zone, whereas EuSAMS1 transcripts were hardly detected. These results suggest different regulatory mechanisms for the two genes in the root nodule. The expression pattern of SAMS genes in the root nodule may correlate mostly with cell wall synthesis, polyamine biosynthesis and other methylation-mediated functions.