Salt stress alters A/T-rich DNA-binding factor interactions within the phosphoenolpyruvate carboxylase promoter from Mesembryanthemum crystallinum.

The common ice plant, Mesembryanthemum crystallinum, shifts from C3 to crassulacean acid metabolism (CAM) photosynthesis in response to osmotic stress. The expression of a number of genes encoding enzymes involved in the CAM pathway increases as a result of increased transcription rates. To begin to investigate the mechanisms responsible for the transcriptional activation, we have characterized the 5' control region of a specific isoform of phosphoenolpyruvate carboxylase gene (Ppc1) that plays a key role in CAM. We have determined the nucleotide sequence of the 5' flanking region of this gene. Ppc1 contains a long 5'-leader sequence with the transcriptional start site located 332/333 nucleotides 5' of the translational initiation codon. Multiple DNA interactions with nuclear factors are detectable within the 5'-flanking region of Ppc1. We have used copper orthophenanthroline footprinting to demonstrate that one particularly abundant factor (designated PCAT-1) binds the Ppc1 promoter at two distinct A/T-rich sites located -128 to -158 and -187 to -205 bp upstream of the transcriptional start site. These binding sites share a loose consensus motif having the sequence AARTAAC(T/A)A(G/T)TTTY. Gel retardation competition experiments with oligonucleotides containing these A/T-rich binding sites suggest that both sites bind the same factor, but with different affinities. Fractionation of crude nuclear extracts by heparin-agarose chromatography indicates that PCAT-1 is more prevalent in extracts prepared from salt-stressed leaf tissue. Additional binding activities that interact with the PCAT-1 binding sites have been detected that either increase or decrease in abundance or binding affinity in response to salt stress.