38th Annual Meeting of the Society for Invertebrate Pathology

The nuclear genome is typically quite spacious, but it has been highly compacted in several cases, most spectacularly in microsporidian parasites. We have conducted genome sequence survey (GSS) and expressed sequence tag (EST) surveys to study effects of this compaction, and two will be discussed here. Mitochondrial proteins are targeted using an N-terminal transit peptide, but we have found that most microsporidian mitochondrial proteins lack such extensions. Nevertheless, using GFP-fusion proteins in yeast we show that microsporidia appear to use a homologous but simplified system of targeting, perhaps using the N-terminus but not as a leader. Gene expression has been studied by conducting the first microsporidian EST sequencing. This survey showed that many transcripts encode fragments of or complete copies of several genes. These multi-gene transcripts are not polycistronic: instead promoters and terminators were squeezed from shrinking intergenic regions into adjacent genes. To determine if this was caused by compaction, we sequenced ESTs from even more highly compacted genomes (nucleomorphs) of endosymbiotic green and red algae. These possess multi-gene transcripts at even higher frequencies. Altogether, compaction may have subtle but important effects on genome functions, and also points to potential challenges in studying expression in these systems (e.g., using arrays), since the actual target of expression can be difficult to discern.