Differential organ-specific expression of three poly(A)-binding-protein genes from Arabidopsis thaliana

Poly(A)-binding protein (PABP) is considered an essential component of a eukaryotic cell; deletion of the PABP-coding gene in yeast leads to a lethal phenotype. PABP is implicated in numerous aspects of posttranscriptional regulation, including mRNA turnover and translational initiation. A nested set of degenerate PCR primers designed from regions conserved among yeast, Xenopus, and human PABP sequences was used to amplify genomic DNA fragments from Arabidopsis thaliana. Hybridization screening of genomic and cDNA libraries with a genomic PCR probe led to the isolation of three diverse Arabidopsis genes encoding PABPs, PAB1, PAB3, and PAB5. All three sequences contain the expected four RNA-recognition motifs. Sequence diversity between these genes equals or exceeds the diversity among animal and fungal sequences. One of the genes, PAB5, and its cDNA were completely sequenced. Its open reading frame encodes a 73.2-kDa protein containing a number of amino acid motifs characteristic of PABPs from different species. Moreover, in vitro synthesized PAB5 protein bound to poly(A)-Sepharose with high specificity. All three genes isolated showed organ-specific patterns of expression. PAB5 and PAB3 RNAs were detected only in floral organs, with the highest level of expression in immature flowers. PAB1 RNA was observed predominantly in roots, was less abundant in immature flowers, and was not detected in any other organ examined (stems, leaves, mature flowers, siliques). This suggests a potentially unique role for PABPs in organ-specific posttranscriptional regulation in plants.