Molecular Characterization of the D Surface Protein Gene Subfamily in Paramecium primaurelia

When Paramecium primaurelia expresses the D serotype, a major high molecular weight mRNA species is detected in the cytoplasm. Using the cDNA derived from this mRNA as a probe, three very similar genes, Dα, Dβ and Dγ, were cloned. Of these three genes, we show that only the Dα mRNA is present in the cytoplasm of cells expressing the D serotype and corresponds to the major mRNA species. The nucleotide sequence of the entire coding region of the Dα gene, as well as the upstream and downstream sequences, has been determined. The 7632‐nucleotide open reading frame encodes a putative protein that displays the characteristic cysteine residue periodicity of Paramecium surface antigens but does not contain central tandemly repeated sequences. Partial sequences of the two nonexpressed genes Dβ and Dγ indicate a high percentage of identity (90%–95%) with the Dα gene, suggesting that Dβ and Dγ genes are either very similar surface protein genes whose transcription is repressed trough mutual exclusion, or perhaps are pseudogenes. A region of variable DNA rearrangement was identified 1 kb upstream of the Dγ gene. This macronuclear region arises from the same micronuclear locus by alternative excision of internal eliminated sequences during macronuclear development.