MOLECULAR CHARACTERIZATION OF THE D-SURFACE PROTEIN GENE SUBFAMILY INPARAMECIUM-TETRAURELIA

When Paramecium tetraurelia expresses the D serotype, detectable by se rum tests, high molecular mRNA could be isolated, which corresponds to the molecular mass of the D surface protein. Using this D specific mR NA as a probe for screenings in different genomic libraries a subfamil y of five very similar genes was found, named alpha-51D, gamma(1)-51D, gamma(2)-51D, delta-51D and epsilon-51D. Each of them is about 8-kb l ong, they show regions of identity to each other, and there is no evid ence that any are defective genes or pseudogenes. Up to now serotype D is the only known serotype showing this phenomenon. Another novel fea ture is that two of the D isogenes are closely linked. The sequence fo r the entire coding region of the alpha-51D gene has been determined, as well as the upstream and downstream noncoding regions. Its deduced amino acid sequence shows the same characteristic cysteine periodicity displayed by all other immobilization antigen (i-ag) genes from Param ecium. However, in contrast to most other such genes, tandem repeats a re missing from the 7599-bp long coding region of the alpha-51D gene. When the sequences of the type 51D genes are compared to each other, t he similarity is very high and extends to coding as well as to noncodi ng regions. Similarity within noncoding regions is usually only observ ed for allelic i-ag genes. We conclude that the type D genes constitut e a family of isogenes that are nonallelic. They contain slightly diff erent consensus sequences with possible functions as regulatory region s.