DST SEQUENCES, HIGHLY CONSERVED AMONG PLANT SAUR GENES, TARGET REPORTER TRANSCRIPTS FOR RAPID DECAY IN TOBACCO

DST elements are highly conserved sequences located in the 3' untranslated regions (UTRs) of a set of unstable soybean transcripts known as the small auxin-up RNAs (SAURs). To test whether DST sequences could function as mRNA instability determinants in plants, a model system was developed to facilitate the direct measurement of mRNA decay rates in stably transformed cells of tobacco. Initial experiments established that the chloramphenicol acetyltransferase (CAT) and beta-glucuronidase (GUS) transcripts degraded with similar half-lives in this system. In addition, their decay kinetics mirrored the apparent decay kinetics of the corresponding transcripts produced in transgenic plants under the control of a regulated promoter (Cab-1). The model system was then used to measure the decay rates of GUS reporter transcripts containing copies of the DST sequence inserted into the 3' UTR. An unmodified CAT gene introduced on the same vector served as the internal reference. These experiments and a parallel set utilizing a beta-globin reporter gene demonstrated that a synthetic dimer of the DST sequence was sufficient to destabilize both reporter transcripts in stably transformed tobacco cells. The decrease in transcript stability caused by the DST sequences in cultured cells was paralleled by a coordinate decrease in transcript abundance in transgenic tobacco plants. The implications of these results for the potential function of DST sequences within the SAUR transcripts are discussed.