Cloning and molecular analysis of cDNAs encoding three sucrose phosphate synthase isoforms from a citrus fruit (Citrus unshiu Marc.)

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Abstract
Three partial cDNA clones (pSPS1, pSPS2 and pSPS3) encoding sucrose phosphate synthases (SPS) were isolated by Reverse Transcription (RT)-PCR using first-strand cDNA prepared from the leaf or fruit of citrus (Citrus unshiu Marc.). The nucleotide and deduced amino acid sequences of the three clones showed significant similarities to SPS previously isolated in other plants. A full-length, cDNA clone, CitSPS1, was isolated from a fruit (juice sacs and pulp segment) cDNA library using one (pSPS1) of the three partial clones as a probe. The 3539-bp CitSPS1 clone coded for a 1057-amino acid polypeptide with a predicted molecular mass of 117.8 kDa. The amino acid sequence deduced from the CitSPS1 clone showed homology with SPS from maize (55.8% identity) and spinach (74.0% identity). Genomic Southern blot analysis suggested that CitSPS1 clone represents a low-copy-number gene. RNA blot analysis of leaf, flower and fruit showed that CitSPS1 and pSPS2 were expressed in all organs. However, the levels of expression of CitSPS1 in young leaves, flowers and immature fruits were low, but high in mature leaves, and fruit. pSPS2 transcripts were barely detectable in young leaves and immature fruits, low in mature leaves, and high in flowers and mature fruits. pSPS3 transcripts were only detected in young and in mature leaves.