EUPLOTES PHEROMONE GENES: STRUCTURALLY SIMPLE STRUCTURE AND COMPLEX EXPRESSION MECHANISMS

Ciliates are unique in possessing two genomes within the same cytoplasm: a chromosomic transcriptionally silent germinal genome residing inside the cell micronucleus and a sub-chromosomic transcriptionally active somatic genome residing inside the cell macronucleus. The latter is anew generated from the former at every sexual event through short-term dramatic phenomena of chromosome fragmentation, DNA elimination and gene amplification. In spirotrichous ciliates such as Euplotes, the macronuclear genome comprises only gene-size DNA molecules characterized by telomere-capped extremities and a single coding region flanked by 5’ and 3’ un-translated regions of variable lengths. We analyzed the expression of these macronuclear genes in two gene families encoding water-borne signaling pheromones responsible for cell-cell communication and recognition phenomena in two common marine species of Euplotes, E. crassus and E. raikovi. Although structurally simple, these genes regulate their expression through mechanisms that are much more complex than previously thought. They result in the synthesis of multiple transcripts generated by the use of different transcription start sites and the removal of intron sequences from the conserved 5’ regions. However, the presence of several in-frame UAA and UAG stop codons positioned far from the gene poly(A) tail raises the question on whether every transcript is actually able to specify a functional product. We are currently verifying the hypothesis that these stop codons are translated, as it occurs in other ciliates, as sense codons specific for the amino acid glutamine.