Tortula ruralis is a desiccation-tolerant bryophyte with a novel post-transcriptional gene response to drying and rehydration. To better understand the molecular basis of mRNA stability and selective mRNA translation upon rehydration, 149 T. ruralis cDNAs were selected and analyzed by sequence comparison for the presence of cis-acting elements associated with the formation of mRNA 3′-ends. The cDNAs analyzed were 140 expressed sequence tags (ESTs) isolated from desiccated gametophytes (AI304967–AI305106), a calmodulin-like domain protein kinase (U82087), the rehydrins Tr155 (U40818) and Tr288 (U21679), and six previously unreported ESTs isolated from desiccated gametophytes (AI665854–AI665859). The majority of cDNAs contained within the 3′ untranslated region a consensus AAUAAA poly (A) signal and a highly conserved far-upstream element (FUE) that was similar to the plant consensus GU-rich sequence. The T. ruralis FUE consensus sequence is WUUUUGUUK (where W = A U and K = G U) which we have designated as the UUG-core motif. We have characterized five classes of cDNA's based upon the modular architecture of the poly (A) signal motif. Each cDNA class, with the exception of class V, contained the UUG-core motif. Class I (18% of the total) contained the conserved poly (A) signal sequence 0–30 nucleotides from the poly (A) site. Class II (28% of the total) contained a consensus hexanucleotide 0–30 nucleotides from the poly (A) site. Class III (2% of the total) contained an exact copy of a related poly (A) signal (AAUGAA) more than 30 nucleotides from the poly (A) site. Class IV (20% of the total) contained a consensus hexanucleotide more than 30 nucleotides from the poly (A) site. Class V (32% of the total) contained no known poly (A) signal. Analysis of the relative frequency of sequences between 1 and 60 nucleotides upstream from the poly (A) tail was determined for each cDNA sequence. Overall, the 60 nucleotide sequence was found to be U-rich and C/G poor with an A-rich region between nucleotides 10 and 30. Putative poly (A) signals are arrayed within the 3′ untranslated region of T. ruralis cDNAs as predicted from the angiosperm model systems, including the absence of a conserved unifying cis-acting motif. To our knowledge, this is the first description of the molecular architecture of poly (A) signals within moss cDNAs.
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Vol. 103 • No. 1