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.