A Tortula ruralis cDNA Rps3a, encoding a predicted polypeptide with significant similarity to the small-subunit ribosomal protein S3a, was isolated from a desiccated gametophyte cDNA library. The deduced 248 amino acid polypeptide is approximately 28 kDa, with a predicted pl of 10.09, and shares extensive identity (> 75%) with S3a ribosomal proteins from Arabidopsis thaliana and Helianthus annuus. The deduced polypeptide (RPS3a) contains three nuclear targeting signals, one of which is unique to the bryophyte S3a homologue, and is strongly predicted by PSORT to be nuclear localized (> 97% certainty). Northern blot hybridization using total and polysomal RNA demonstrated Rsp3a is constitutively expressed in moss gametophytes during a wet/dry/wet cycle. Phylogenetic analysis of the deduced amino acid sequence demonstrates T. ruralis RPS3a is most similar to S3a ribosomal proteins from Tracheophytes. Model plants, such as Arabidopsis thaliana (Somerville & Meyerowitz 1994), have been powerful experimental tools for the elucidation of complex biological processes. However, the analysis of several important plant phenotypes, most notably the ability to tolerate extreme environmental conditions, has been difficult using these models because the traits are either not clearly expressed or entirely lacking. Bryophytes that exhibit many of these important tolerant phenotypes offer realistic models for the analysis of environmental stress-tolerance. Model bryophyte systems, such as the desiccation-tolerant moss Tortula ruralis, have proven extremely useful in the study of vegetative desiccation-tolerance and post-transcriptional gene control (Bewley 1979, 1995; Oliver & Bewley 1997; Oliver & Wood 1997; Oliver et al. 1997, 1998). Analysis of organisms from these ancient clades will provide greater insight into the stress induced cellular responses of plants (Oliver & Wood 1997) and may provide unique genetic material for the enhancement of stress-tolerance within economically important angiosperms. The desiccation-tolerance ability of T. ruralis is afforded by two integrated processes: a constitutive protection system and an active rehydration induced recovery mechanism apparently unique to ryophytes (Oliver & Bewley 1997; Oliver & Wood 1997). As such, we hypothesize that genes essential to recovery and cellular repair are prefe entially expressed upon rehydration of desiccated gametophytes, and genes that are essential to limiting cellular damage are expressed under wet, drying, and rehydration conditions (Oliver & Bewley 1997; Oliver & Wood 1997; Oliver et al. 1998; Scott & Oliver 1994). In order to gain a more complete understanding of the genes involved in desiccation-tolerance, we recently characterized 152 expressed sequence tags (ESTs) derived from polysomal RNA isolated from desiccated T. ruralis gametophytes by the single pass sequencing of randomly selected clones (Wood et al. 1999). Seven of these ESTs encode either small (S) or large (L) ribosomal proteins (r-proteins) that are key structural components of a functional ribosome and contribute to the proper and efficient translation of mRNAs (Moore 1998). Plant r-proteins are categorized using a unified system of nomenclature that is based upon rat r-protein designations (BailleySerres 1998; Wool et al. 1991). There are estimated to be 78 r-proteins in rat, all with an apparent molecular mass 8.5 (Wool et al. 1995). Plant r-proteins have simPresent address: Department of Biology, University of Akron, Akron, OH 44325-3908, U.S.A. 2 The nucleotide sequence data will appear in EMBL, GenBank and DDBJ Nucleotide Sequence Databases under the accession number AFO 93109. 3To whom correspondence should be addressed: wood @plant.siu.edu 0007-2745/99/418-425$0.95/0 This content downloaded from 157.55.39.35 on Fri, 02 Sep 2016 04:04:49 UTC All use subject to http://about.jstor.org/terms 1999] DUFF ET AL.: TORTULA RURALIS cDNA 419 ilar biophysical properties. However, due to the lack of a systematic study within a single model plant, the number of r-proteins in angiosperms is estimated to range from 75 to 92 polypeptides (Bailley-Serres 1998). We describe the isolation and characterization of a cDNA from the bryophyte Tortula ruralis encoding a polypeptide with significant similarity to ribosomal protein S3a. The r-protein S3a has been demonstrated by cellular co-purification experiments to be associated with the 40S ribosomal subunit in Drosophila melanogaster (Reynaud et al. 1997) and similar plant cDNAs have been identified in A. thaliana (Newman et al. 1994), rice (Kidou et al. 1994) and Catharanthus roseus (Ito et al. 1991). To our knowledge, T. ruralis Rps3a is the first ribosomal protein cDNA described from a bryophyte system and as such we also present a phylogenetic analysis of the deduced amino acid