POSTTRANSCRIPTIONAL REGULATION OF A SALT-INDUCIBLE ALFALFA GENE ENCODING A PUTATIVE CHIMERIC PROLINE-RICH CELL-WALL PROTEIN

A cDNA previously shown to identify a salt-inducible root-specific tra nscript in Medicago saliva was used to screen an alfalfa library for t he corresponding genomic sequence. One positive clone was recovered. T he nucleotide sequence of a subclone contained a 329 bp 5' region upst ream of the first ATG codon, a 1143 bp coding segment, and a 447 bp 3' -untranslated region interrupted by a single 475 bp intron. Translatio n of the coding segment, which was designated MsPRP2, suggested it enc odes a chimeric 40569 Da cell wall protein with an amino-terminal sign al sequence, a repetitive proline-rich sequence, and a cysteine-rich c arboxyl-terminal sequence homologous to nonspecific lipid transfer pro teins. The 3'-untranslated region of MsPRP2 contained a sequence simil ar to one found to destabilize mRNAs transcribed from the elicitor-reg ulated proline-rich protein gene PvPRP1. Transcription run-on experime nts using nuclei from salt-sensitive and salt-tolerant alfalfa callus suggested that the accumulation of MsPRP2 transcripts in salt-tolerant alfalfa cells grown in the presence of salt is due primarly to increa sed mRNA stability. The MsPRP2 gene thus may be a useful model for stu dying post-transcriptional salt-regulated expression of cell wall prot eins.