A cDNA clone for isocitrate lyase from tomato.

The glyoxylate cycle enzyme ICL (EC 4.1.3.1) cleaves isocitrate into succinate and glyoxylate. The glyoxylate cycle converts acetyl COA, derived from P-oxidation of fatty acids, into C, intermediates of the tricarboxylic acid cycle (Huang et al., 1983). Expression of the glyoxylate cycle enzymes in plants has been associated with mobilization of storage lipids in seeds and cotyledons and during leaf senescence (Turley and Trelease, 1990; Pistelli et al., 1991; Zhang et al., 1993). To isolate nonphotosynthetic genes highly expressed specifically in leaf tissue, a cDNA library was constructed from mRNA isolated from dark-treated first true leaves of tomato (Lycopersicon esculentum cv UC82B) and was differentially screened. In this paper, we report the complete sequence of one of these clones (Tlsl), which has greater than 70% nucleotide identity with ICL genes from cotton, castor bean, soybean, Brassica napus, and Arabidopsis (Table I). Comparison of the deduced amino acid sequence of Tlsl with sequences from plants, filamentous fungi, yeast, and bacteria indicates that this enzyme is highly conserved across phyla. Tlsl contains the putative active site sequence KKCGHM that is conserved between a11 ICL genes as well as the C-terminal tripeptide ARM thought to be involved in targeting to the glyoxysomes. Northern analysis has shown that in tomato high steady-state levels of ICL mRNA are present in cotyledons, first true leaves, and mature leaves of glasshouse-grown plants. Weak expression was observed in senescent leaves, but no detectable expression was observed in root or fruit tissue. Table 1. Characteristics of Tlsl