Survey of transcription initiation in the streamlined genomes of Paramecium

The compact macronuclear genome of the genus Paramecium is optimized for efficient gene expression. To study transcription initiation in this streamlined genome and understand the influence of sequence architecture on gene expression and loss, researchers examined the distribution and diversity of candidate transcription initiation sites (TISs) in Paramecium sexaurelia, Paramecium tetraurelia, and their closest relative, Paramecium caudatum.

The study found that Paramecium genes typically have very short 5 prime untranslated regions (UTRs) of 40 base pairs or less, and their TIRs exhibit a median dispersion ranging from 8 to 10 base pairs. These TIRs exhibit high AT content.

Interestingly, while TIR dispersion does not correlate with gene expression, the mean TIS position relative to the translation start site of each gene does impact gene expression in Paramecium sexaurelia, Paramecium tetraurelia, and Paramecium caudatum. This relationship is often conserved across the three species. However, it is gene expression, not the mean TIS position or TIR dispersion, that primarily drives the retention of paralogs in the aurelia species.

Compared to other eukaryotes, Paramecium exhibits a distinctly well-defined and short main TIS region. Additionally, unique sequence motifs in Paramecium likely differ from the consensus found in multicellular eukaryotes.