The immobilization antigens (i-antigens) of Paramecium, large polypeptides of relative molecular mass approximately 300,000, are located on the cell surface. Each i-antigen is encoded by a different unlinked gene, and no more than one gene is expressed at a time. The proteins and the mRNAs and genes encoding them are readily isolated. Here we report the nucleotide sequence of three regions of the A i-antigen gene from stock 51 of Paramecium tetraurelia. Surprisingly, all reading frames contain TAA and TAG stop codons, even though there is evidence that one reading frame of these sequences codes for the i-antigen. We suggest that in Paramecium UAA and UAG code for amino acids, instead of serving as translational stops as they do in all other organisms.

Surface proteins from 11 antigenic types of Paramecium tetraurelia vary in molecular weight from 251,000 to 308,000. The size of a series of polyadenylated RNAs obtained from these types were correlated with the sizes of the proteins and judged to be the mRNAs for the proteins. The mRNAs were used to identify genomic DNA clones containing complementary sequences. The gene for antigen A was present in one copy per genome, and the data suggest that extensive introns were absent. When restriction enzyme digests of DNA from cultures of paramecia with active and inactive genes were probed with portions of the cloned genes, no evidence for rearrangements or changes in gene dosage was found.

Immobilization antigens of stock 51 of Paramecium tetraurelia were subjected to electrophoresis in NaDodSO4/ polyacrylamide gels. Type A is estimated to have a molecular size of 300,000 daltons; H is estimated to be 288,000, D to be 280,000, E to be 270,000, B to be 253,000, and C to be 250,000. Poly(A)+RNAs have been isolated from cells producing these antigens and subjected to electrophoresis in methylmercury gels. A major band is found to vary in mobility with antigenic type: Its position in preparations derived from paramecia synthesizing antigen A indicates a size of 8400 nucleotide residues; its position from paramecia synthesizing other antigens indicate H, 8200; D, 7900; E, 7500; B, 7600; and C, 7000. Because ofthe sizes and quantities of these RNAs, it is argued that they probably represent the mRNAs for the immobilization antigens. It is concluded that each immobilization antigen probably consists of a single polypeptide and that only one major serotype-determining mRNA is present in each antigenically different paramecium.Differences in gene expression that can be classified as simple environmental modifications are well known. Often, however, differences in gene expression show cellular inheritance; subclones that have alternative phenotypes may arise and reproduce true to type, even though all are under the same environmental conditions. The molecular mechanisms responsible for several instances ofinherited differences in gene expression have recently been discovered. Gene rearrangements have proved to be the basis for all cases. They include mating types in yeast (1), antigenic variation in Salmonella (2) and trypanosomes (3-5), and the production ofantibodies (6). Transposable elements in a variety of organisms (7-9) can also lead to modification of gene action.Since all ofthese cases involve changes in DNA, it is not surprising that they all are relatively stable and are not, as far as we know, directed by environmental stimuli. The production of antibodies may be an exception. Although antigens trigger the formation of specific antibodies, they do so by stimulating growth and synthesis. They select cells that have specific rearrangements, but there is no evidence that they induce rearrangements. Moreover, order in the sequence of changes exhibited by classes ofantibodies and in the succession ofantigenic types in trypanosomes does not necessarily imply specific induction. Nevertheless, a role for the environment in the induction of antibodies may exist, for the rearrangements occur in specific cells at specific times in development, suggesting that unknown inducing conditions may exist in those cells.Differences in the expression of genes determining immobilization proteins or i antigens (i-ags) of Paramecium are in some respects anomalous. They are heritable under certain environmental conditions and some show preferred sequences of change from one state to the other. Brown (10) has suggested that they too will prove to be due to gene rearrangments. However, they are unlike the cases d...

A family of genes is responsible for production of surface antigenic components of Paramecium tetraurelia. These surface proteins are expressed in a mutually exclusive manner. Individuals rarely display more than one type. However, changes in environmental conditions can cause different surface proteins which replace preexisting types to be expressed. We investigated the nature of regulation of the genes for the A, C, and H surface antigens of P. tetraurelia. A system for in vitro run-on transcription was developed from crude Paramecium extracts and used in this analysis. The genes for surface antigens A and H were controlled at the level of transcription. However, the gene for surface antigen C demonstrated both transcriptional and posttranscriptional control, depending on the serotype being expressed. When animals expressed serotype A, the gene for surface antigen C was not transcribed. However, when animals expressed serotype H, the gene for surface antigen C was actively transcribed and stable surface antigen C mRNA was present in the cells, although surface antigen C was not detectable by serotype testing or by a salt-alcohol extraction method. The kinetics of transformation from serotype H to serotype C were determined by using the in vitro transcription system and monitoring steady-state RNA levels. During the transition, serotype A transcription was detected in run-on transcription experiments, although this RNA did not accumulate. The results indicate that serotype expression is controlled at several levels and that not all serotype genes are controlled in the same manner.

When stock 16 sensitives are exposed to kappa particles of stock 7, the kappa particles enter food vacuoles i n large numbers. The kappa quickly break down, R bodies unroll and penetrate the cytoplasm, food vacuole membranes show a general breakdown, and the formation of new food vacuoles is suppressed. The observations suggest that R body unrolling is important i n the toxic activity and that the food vacuole membranes may be a major site of action of the toxin.