Several of the introductory essays in this blog have dealt with aspects of polyadenylation and the complex that mediates this process. As shown in this figure and as discussed (in part) here and here, the complex is sizeable and possesses a number of activities, including some that seem superfluous. However, genetic studies in yeast indicate that virtually every subunit of the core complex is essential – for viability and for pre-mRNA processing and polyadenylation in vitro and in vivo. Biochemical and/or immunological depletion studies reveal a similar scenario in mammals, and a less-expansive set of studies suggests that a similar rule of thumb will apply in plants. The bottom line of all of this is that almost all of the subunits of the polyadenylation complex seem to be essential. In the vernacular of a proponent of intelligent design, the polyadenylation complex would seem to be irreducibly complex.
It is in this context that the recently-completed genome of the parasitic organism Giardia lamblia enters the fray. Last year, the complete sequence of G. lamblia, some 12 million base pairs, was determined and analyzed. The authors of the study published in Science noted a number of interesting things – a preponderance of genes encoding protein kinases, evidence for substantial horizontal gene flow from bacteria and archaebacteria, and a streamlined core gene expression machinery (transcription and RNA processing). This streamlining is especially notable in the case of the polyadenylation machinery. Remarkably, of all the subunits pictured in this figure, genes for only three* can be found in G. lamblia (see the figure that follows this paragraph).