The translational apparatus of Tortula ruralis: polysomal retention of transcripts encoding the ribosomal proteins RPS14, RPS16 and RPL23 in desiccated and rehydrated gametophytes

Tortula ruralis (Syntrichia ruralis) is an important model system for the s tudy of plant vegetative desiccation tolerance. One of the most intriguing aspects of desiccation-tolerant plants is the maintenance of key cellular c omponents in stable and viable forms in the desiccated state, particularly those related to the translational apparatus (i.e. ribosomes and ribosomal RNAs), This study investigated the third integral component of the translat ional apparatus, the ribosomal proteins, Three T. ruralis cDNAs encoding pr edicted polypeptides with significant similarity to ribosomal proteins were isolated from a cDNA expression library derived from the polysomal, messen ger ribonucleoprotein particle (mRNP) fraction of desiccated gametophytes; Rps14 and Rps16 encode the small-subunit ribosomal proteins RPS14 and RPS16 , respectively, and RpI23 encodes the large-subunit ribosomal protein RPL23 . RPS14, RPS16 and RPL23, the deduced polypeptides, have predicted molecula r masses of 14.4 kDa, 16.2 kDa and 14.9 kDa and predicted pI's of 11.08, 10 .34 and 10.67, respectively. Phylogenetic analysis of the deduced amino aci d sequences demonstrated that each of the T. ruralis proteins is most simil ar to ribosomal proteins from higher plants even though RPS14 and RPL23 sho w high divergence from their other plant counterparts. RNA blot hybridizati ons of RNAs present within the polysomal mRNP fraction (i.e. the 100 Kxg pe llet) demonstrated that Rps14, Rps16 and Rp123 are expressed in moss gameto phytes during a desiccation-rehydration cycle and, according to the prior c DNA classification scheme in T. ruralis, are constitutive clones. These fin dings clearly demonstrated that Rps14, Rps16 and Rp123 transcripts are reta ined within the polysomal fractions of desiccated gametophytes.