Arabidopsis Gene and cDNA Encoding Cell-Wall Invertase

Suc is the final product of photosynthesis, but before it can be utilized it must be cleaved into hexoses either by invertase (0-fructofuranosidase, EC 3.2.1.26) or by SUC synthase (EC 2.4.1.13). There are three recognized types of invertase present in plant cells: soluble neutra1 invertase, soluble acid invertase, and insoluble cell-wall acid invertase. Invertase genes encoding cell-wall and vacuolar (soluble) acid invertases have been characterized from Daucus carota (Ramloch-Lorenz et al., 1993) and from Lycopersicon (either esculentum or pimpinellifollium, Elliott et al., 1993), respectively. So far, no invertase gene from Arabidopsis thaliana has been isolated. We report here the characterization of a cell-wall invertase gene from A. thaliana and its cognate cDNA. A genomic fragment containing Atbfructl was identified by screening a genomic library (EMBL3, Clontech, Palo Alto, CA) with a 1kb fragment from a cDNA encoding a cell-wall invertase in D. carota (Sturm and Chrispeels, 1990). The Atbfructl cDNA clone was identified by screening an A. thaliana cDNA library with exon 3 of the Atbfructl gene. The genomic clone Atbfructl is 4237 bp in size (Table I). Alignment of the Atbfructl gene sequence with that from its cognate cDNA showed the presence of seven exons. The organization of the gene is similar to that of invertase genes in D. carota (Ramloch-Lorenz et al., 1993) andL. esculentum (Elliott et al., 1993), but the size of the introns is significantly smaller in A. thaliana. In Atbfructl and both tomato genes, exon 2 is only 9 bp long and encodes part of a highly conserved region found in a11 known invertase proteins (NDPNG). By way of contrast, in the D. carota gene this short nucleotide sequence is included in exon 1. There is one conflict between the gene sequence and the cDNA sequence: the third base in the codon coding for ArgS1' is a T in the gene and a C in the cDNA. The cDNA clone Atbfructl is 1947 bp in size with an open reading frame of 1755 bp coding for a protein of 584 residues.