IDENTIFICATION OF ENHANCER AND SILENCER REGIONS INVOLVED IN SALT-RESPONSIVE EXPRESSION OF CRASSULACEAN ACID METABOLISM (CAM) GENES IN THE FACULTATIVE HALOPHYTE MESEMBRYANTHEMUM-CRYSTALLINUM

In response to salinity or drought stress, the facultative halophyte M esembryanthemum crystallinum will switch from C-3 photosynthesis to Cr assulacean acid metabolism (CAM). During this switch, the transcriptio n rates of many genes encoding glycolytic, gluconeogenic, and malate m etabolism enzymes are increased. In particular, transcription of the P pc1 and Gap1 genes encoding a CAM-specific isozyme of phosphoenolpyruv ate carboxylase and NAD-dependent glyceraldehyde-3-phosphate dehydroge nase, respectively, is increased by salinity stress. To investigate th e molecular basis of salt-induced gene regulation, we examined the Ppc 1 and Gap1 promoters for cia-elements and trans-acting factors that ma y participate in their expression. Ppc1 or Gap1 promoter-beta-glucuron idase chimeric gene constructs containing various deletions were intro duced into intact, detached M. crystallinum leaves by microprojectile bombardment. The Ppc1 5'-flanking region contains several salt-respons ive enhancer regions and one silencer region reflecting the complex re gulation patterns exhibited by this promoter in vivo. A region localiz ed between nucleotides -977 and -487 relative to the transcriptional s tart site appears to regulate the magnitude of salt-inducibility. In c ontrast, the Gap1 promoter contains a single region from -735 to -549 that confers salt-responsive gene expression. Alignment of these 5'-fl anking regions reveals several common sequence motifs that resemble co n sensus binding sites for the Myb class of transcription factors. Ele ctrophoretic gel mobility shift assays indicate that both the -877 to -679 region of Ppc1 and the -735 to -549 region of Gap1 form a DNA-pro tein complex unique to nuclear extracts from salt-stressed plants. The appearance of this DNA-protein complex upon salt stress suggests that it may participate in salt-induced transcriptional activation of Ppc1 and Gap1.