Poly(A) polymerases - an interesting gene family

We have identified and partially purified a poly(A) polymerase from nuclei isolated from pea leaves (1), and we have identified Arabidopsis and rice genes whose predicted products are similar to mammalian polyadenylation factor subunits (including the poly(A) polymerase). Interestingly, the Arabidopsis CPSF100 homolog has been found to interact with at least one of the four Arabidopsis PAPs (2). At least one of the four PAPs also interacts with an Arabidopsis Fip1 ortholog (4), such that the Fip1 protein stimulates the non-specific polyadenylation activity of the purified recombinant protein. The latter results are consistent with reports of the yeast and mammalian Fip1 proteins, and suggest that Fip1 in plants serves to recruit an active PAP to the processed pre-mRNA.

The Arabidopsis genome has four genes that encode poly(A) polymerase isoforms (3, 5). One of the four isoforms is much smaller than the other three, lacking the RNA-binding domain and nuclear localization signals that are hallmarks of canonical poly(A) polymerases (including the other three Arabidopsis isoforms). Remarkably, RNAs from all four genes are alternatively-spliced near the 5'-end of their respective transcription units, so that mRNAs encoding highly-truncated polypeptides are produced. This alternative splicing varies from tissue to tissue; along with tissue specificity in the expression of the four genes, this suggests that different poly(A) polymerase isoforms may play specific roles during plant development. This suggestion is supported by the observation that T-DNA insertions in each of the four Arabidopsis PAP genes are lethal (5).


1. Hunt AG, Meeks LR, Forbes KP, Das Gupta, Mogen BD. 2000. Nuclear and chloroplast poly(A) polymerases from plants share a novel biochemical property. Biochemical and Biophysical Research Communications 272, 174–181.

2. Elliott BJ, Dattaroy T, Meeks-Midkiff LR, Forbes KP, Hunt AG. 2003. An interaction between an Arabidopsis poly(A) polymerase and a homologue of the 100 kDa subunit of CPSF. Plant Mol. Biol. 51:373-84

3. Addepalli B, Meeks LR, Forbes KP, Hunt AG. 2004. Novel alternative splicing of mRNAs encoding poly(A) polymerases in Arabidopsis. Biochimica Biophysica Acta 1679: 117-128.

4. Forbes KP, Addepalli B, Hunt AG. 2006. An Arabidopsis Fip1 homologue interacts with RNA and provides conceptual links with a number of other polyadenylation factor subunits. J. Biol. Chem. 281: 176-186.

5. Meeks, LR. 2005. Isolation and characterization of the four Arabidopsis thaliana poly(A) polymerase genes. Ph.D. thesis, University of Kentucky, 2005.