Programmed DNA under-amplification in Paramecium primaurelia

Ciliates are unicellular eukaryotic organisms that contain two types of nuclei throughout their vegetative life, transcriptionally active macronuclei governing the cell phenotype, and transcriptionally inert micronuclei. Following sexual reproduction, new macronuclear genomes regularly develop from micronuclear genomes through programmed DNA rearrangements that include DNA splicing, DNA fragmentation and DNA amplification. In the course of characterization of the micronuclear version of the 9.0 kb G gene, which encodes the G surface antigen in Paramecium primaurelia, we characterized a G gene duplicate. Compared with the G gene, the G gene duplicate displays features identifying it as a psi G pseudogene. About 1.6 kb upstream from the G gene, we characterized a new gene, the P gene. A related psi P putative pseudogene lies 1.6 kb upstream from the psi G pseudogene, showing that the duplicated region extends over > 15 kb and putatively defining it as a pseudogene region. Within macronuclear genomes, this region is highly under-amplified; its level never exceeds 20% of that of the corresponding G gene/P gene region in the 11 cell clones we tested. Under-amplification of the psi G pseudogene/psi P putative pseudogene region could be due to its distal position on macronuclear chromosomes, the use of alternative DNA fragmentation domains being frequent in Paramecium species, or to intrinsically lower amplification of a large genomic region. Therefore, the psi G pseudogene/psi P putative pseudogene region presented in this study provides a useful tool for the analysis of DNA fragmentation and/or amplification in eukaryotic genomes.