CLONING AND MOLECULAR ANALYSIS OF CDNAS ENCODING 3 SUCROSE-PHOSPHATE SYNTHASE ISOFORMS FROM A CITRUS-FRUIT (CITRUS-UNSHIU MARC)

Three partial cDNA clones (pSPS1, pSPS2 and pSPS3) encoding sucrose ph osphate synthases (SPS) were isolated by Reverse Transcription (RT)PCR using first-strand cDNA prepared from the leaf or fruit of citrus (Ci trus unshiu Marc,), The nucleotide and deduced amino acid sequences of the three clones showed significant similarities to SPS previously is olated in other plants. A full-length, cDNA clone, CitSPS1, was isolat ed 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 clo ne coded ibr a 1057-amino acid polypeptide with a predicted molecular mass of 117.8 kDa. The amino acid sequence deduced rom the CitSPS1 clo ne showed homology with SPS from maize (55.8% identity) and spinach (7 4.0% identity). Genomic Southern blot analysis suggested that CitSPS1 clone represents a low-copy-number gene. RNA blot analysis of leaf, fl ower and fruit showed that CitSPS1 ans pSPS2 were expressed in all org ans. However, the levels of expression of CitSPS1 in young leaves, flo wers and immature fruits were low, but high in mature leaves, and frui t. pSPS2 transcripts were barely detectable in young leaves and immatu re fruits, low in mature leaves, and high in flowers and mature fruits . pSPS3 transcripts were only detected in young and in mature leaves.