ENZYMATIC-PROPERTIES OF CHORISMATE SYNTHASE ISOZYMES OF TOMATO (LYCOPERSICON-ESCULENTUM MILL)

Three plastidic chorismate synthase isozymes (CS1, CS2 and CS2 Delta) of tomato were identified by isolation of the corresponding cDNAs. The se three cDNAs are derived from only two genes (LeCS1 and LeCS2). This additional complexity results from differential splicing of the prima ry transcript of one of the genes (LeCS2) giving rise to two different transcripts (CS2 and CS2 Delta transcripts). All three isozymes were individually expressed in Escherichia coli both as precursor proteins with N-terminal transit peptides and as mature proteins. Only the matu re but not the precursor isozymes CS1 and CS2 were enzymatically activ e. The enzyme CS2 Delta was unstable in E. call. Both CS1 and CS2 were purified to near homogeneity and their enzymatic properties were anal yzed. They differ substantially in their K-m values for the substrate 5-enolpyruvylshikimate 3-phosphate (11 and 80 mu M for the mature form s of CS1 and CS2, respectively). The two isozymes appear to be active only as oligomers, and the potential physiological implications of thi s result are discussed.