2 RELATED LOW-TEMPERATURE-INDUCIBLE GENES OF ARABIDOPSIS ENCODE PROTEINS SHOWING HIGH HOMOLOGY TO 14-3-3-PROTEINS, A FAMILY OF PUTATIVE KINASE REGULATORS

We have isolated two Rare Cold-Inducible (RCI1 and RCI2) cDNAs by scre ening a cDNA library prepared from cold-acclimated etiolated seedlings of Arabidopsis thaliana with a subtracted probe. RNA-blot hybridizati ons revealed that the expression of both RCI1 and RCI2 genes is induce d by low temperature independently of the plant organ or the developme ntal stage considered. However, RCI1 mRNA accumulates faster and at hi gher levels than the RCI2 one indicating that these genes have differe ntial responsiveness to cold stress. Additionally, when plants are ret urned to room temperature, RCI1 mRNA decreases faster than RCI2. In co ntrast to most of the cold-inducible plant genes characterized, the ex pression of RCI1 and RCI2 is not induced by ABA or water stress. The n ucleotide sequences of RCI1 and RCI2 cDNAs predict two acidic polypept ides of 255 and 251 amino acids with molecular weights of 29 and 28 kD a respectively. The alignment of these polypeptides indicates that the y have 181 identical amino acids suggesting that the corresponding,gen es have a common origin. Sequence comparisons reveal no similarities b etween the RCI proteins and any other cold-regulated plant protein so far described. Instead, they demonstrate that the RCI proteins are hig hly homologous to a family of proteins, known as 14-3-3 proteins, whic h are thought to be involved in the regulation of multifunctional prot ein kinases.