HIGH-LEVEL TUBER EXPRESSION AND SUCROSE INDUCIBILITY OF A POTATO SUS4SUCROSE SYNTHASE GENE REQUIRE 5'-FLANKING AND 3'-FLANKING SEQUENCES AND THE LEADER INTRON

The 3.6 kb of 5' flanking sequence, leader intron, and 0.7 kb of 3' se quence from the potato sucrose synthase gene Sus4-16 are sufficient to direct high-level expression in developing tubers, in basal tissues o f axillary buds and shoots, and in meristems and caps of roots, and to confer sucrose inducibility in leaves. By examining a series of delet ion and substitution constructs in transgenic potato plants, we found that this pattern of expression requires 5' flanking sequences both up stream and downstream of position -1500 and that sequences between pos itions -1500 and -267 are essential for sucrose induction. Replacement of the native 3' sequence with the nopaline synthase 3' sequence resu lted in the loss of sucrose inducibility and of expression in basal ti ssues of axillary buds, A general decrease in expression in other tiss ues was also observed. Removal of the 1612-bp leader intron also had a dramatic effect on both the pattern and level of expression.