ENVIRONMENTAL STRESS-MEDIATED DIFFERENTIAL 3' END FORMATION OF CHLOROPLAST RNA-BINDING PROTEIN TRANSCRIPTS

We report the characterization of transcripts from the halophyte, Mese mbuyanthemum crystallinum, encoding a protein with high homology to ch loroplast RNA-binding proteins (cRBP). In this plant chloroplast-relat ed functions are largely protected against salt stress. cRBP transcrip ts are derived from a single gene, Mc32crbp, although three size class es of polyadenylated mRNAs are detected. Transcription rate and steady state amounts of mRNA are developmentally regulated and light control led with strong transcriptional activity as functional chloroplasts ar e established, and with lower maintenance activity thereafter. Upon sa lt stress, the rate of transcription decreases, although transcript le vels increase. Accompanying stress, a change in the distribution of tr anscript size classes is observed as the longest transcript with an un translated 3' end of 381 nucleotides increases relative to transcripts with shorter 3' ends. The long transcript is characterized by the pre sence of five sequence elements in the 3'-untranslated region that are present in cRBP mRNAs from a variety of plants, although not all elem ents are found in each mRNA. The results may indicate a mechanism by w hich mRNA levels of constitutively light-regulated genes may be modula ted without enhanced transcription in response to environmental cues.