Among the conserved proteins of the polyadenylation complex, seen in all eukaryotes (including the highly-reduced polyadenylation complex in Giardia) is the enzyme that adds the poly(A) tail – polynucleotide adenylyltransferase or, more colloquially, poly(A) polymerase. One would think that the evolutionary history of such a core component of the gene expression machinery would be rather unremarkable – it should be present at the outset and pretty much conserved throughout evolutionary history.
Of course, reality is much more interesting. A former student of mine did her thesis on Arabidopsis poly(A) polymerases, characterizing the four (4!) genes and the protein isoforms. A former postdoc in the lab had done some work in rice poly(A) polymerase genes, and found an equally interesting multiplicity of genes as well as some fascinating expression characteristics. This work has been recently published in PLoS ONE; as is my custom, this post is intended to point out the paper and invite (here or at the journal’s site for the paper) comment, discussion, and criticism.
A brief recap and one or two of the more provocative findings:
Higher plants possess poly(A) polymerase gene families that number between 4 and 5 (or more; although not in the paper, plants whose genomes have undergone recent duplications may have twice these numbers). Interestingly, all of the plant genes arose from a single ancestral gene, based on the highly-conserved positions of introns. These positions are different from those seen in animal poly(A) polymerase genes (and, curiously enough, from those seen in the single Chlamydomonas poly(A) polymerase gene).
The results obtained with Arabidopsis mutants indicates that all of the expressed plant isoforms are essential for viability. This and the expression studies performed with rice and Arabidopsis are most consistent with idea that the different isoforms have undergone a degree of functional specialization.
One of the plant isoforms is a cytoplasmic enzyme. The gene encoding the Arabidopsis cytoplasmic poly(A) polymerase is strongly pollen-specific. These characteristics are reminiscent of a mammalian poly(A) polymerase isoform. However, the data in this paper indicate that the mammalian and plant cytoplasmic poly(A) polymerases have independent evolutionary histories; in other words, the occurrence of cytoplasmic poly(A) polymerases in plants and animals is a case of convergent evolution.
Here is Figure 6 from the paper – my attempt to illustrate the history of these genes in the plant lineage.
Meeks LR, Addepalli B, Hunt AG. 2009. Characterization of Genes Encoding Poly(A) Polymerases in Plants: Evidence for Duplication and Functional Specialization. PLoS ONE 4(11): e8082. doi:10.1371/journal.pone.0008082.