ARABIDOPSIS-THALIANA EXPRESSES 3 DIVERGENT SRP54 GENES

The Arabidopsis thaliana Srp54 gene family was determined to consist o f three genes, all of which were cloned and sequenced. In addition, cD NAs corresponding to two of the genes were obtained. To our knowledge this is the first description of multiple Srp54 genes within an organi sm. In contrast to the situation in mammals, where there are only thre e amino acid differences between the mouse and canine sequences, there was significant amino acid sequence diversity among the genes, partic ularly in the methionine-rich region of the protein, which is the regi on responsible for binding to the 7S RNA of the signal recognition par ticle and to the signal sequence of newly synthesized proteins. The am ino acid sequences of the GTP-binding domains of the three clones were 86% identical, whereas the methionine-rich domains were only 65% iden tical. RNA gel blots of various tissues and developmental stages hybri dized with gene-specific probes revealed that all three genes were exp ressed in all the tissues investigated. There were, however, quantitat ive differences in expression levels.