The 5 ' untranslated region of the At-P5R gene is involved in both transcriptional and post-transcriptional regulation

The steady-state level of transcripts coding for the pyrroline-5-carboxylat e reductase of Arabidopsis (At-P5R) increased under salt and heat stress, m ainly because of an enhanced mRNA stability. However, the M-PSR protein lev el was not induced, and its translation was inhibited at initiation stage a nd probably also at later stages. Replacement of the 5' untranslated region (5'UTR) and beta -glucuronidase (gus) fusion analysis revealed that the fi rst 92 bp region of the At-P5R 5'UTR was sufficient to mediate transcript s tabilization and translation inhibition during salt and heat stresses. Furt hermore, the first 92 bp region of the At-P5R 5'UTR was also involved in tr anscription efficiency in a promoter-dependent manner. The results demonstr ated that the stress regulation of At-P5R is complex and involves the 5'UTR which acts at three levels, partly in opposing directions.