With Micronuclear Genome Rearrangements

The micronuclear genome of Tetrahymena thermophila contains Tel-1 elements that structurally resemble transposons. Here we present molecular evidence that Tel-1 transposon-like elements are mobile. The arrangements of Tel-1 elements in the micronuclear genomes of several T. thermophila strains and cell lines were assayed by Southern blotting. The molecular evidence for Tel-1 transposition is most striking in strains that have undergone unusual laboratory-induced meioses. The genetic history of the strains exhibiting evidence of Tel-1 transposition is consistent with periods of genome restructuring in response to genomic “shock” that B. McClintock has suggested could result in transposon activation. M OBILE genetic elements (transposons) were first described by B. MCCLINT~CK as the agents of unstable mutations in maize (reviewed in FEDEROFF 1983). Transposons have now been described in every organism from which they have been actively sought, and have been domesticated as genetic and molecular tools in many organisms. For example, engineered P elements are widely used as transformation vectors in Drosophila (RUBIN and SPRADLINC 1982). It is hoped that the identification of mobile elements in the micronuclear genome of Tetrahymena thermophila will lead to the development of vectors for germline transformation of this ciliated protozoan. In the germline nucleus of T. thermophila, a family of elements called the Tel-1 family has previously been identified (CHERRY and BLACKBURN 1985). However, the mobility of Tel-l elements was not addressed. Here, we address this question using T. thermophila strains in whose genomes transposons may have been activated, because their genetic histories are consistent with periods of genomic restructuring. MCCLINTOCK (1 984) has suggested that transposons are activated in such genomes in response to genomic “shock.” She defined genomic shock as conditions that “force the genome to restructure itself in order to overcome a threat to its survival” (MCCLINTOCK 1984). Genetic analysis in other organisms has shown that movement of transposable elements often results in frequent and unstable mutations (for review see BERG and HOWE 1989). However, any such mutation that is lethal will not be recoverable for analysis. The genetic organization of T. thermophila allows transposition events in the micronuclear genome to occur ’ Present address: Department of Microbiology and Immunology, University of California, San Francisco, California 94143. Genetics 128: 57-67 (September, 1991) without immediate phenotypic consequences. Although unicellular, T. thermophila has two functionally distinct nuclei. The germline micronucleus contains five diploid pairs of chromosomes. The somatic macronucleus is derived from a division product of the micronucleus following mating. It contains a highly processed and amplified version of the micronuclear genome. Although there is one reported case of transcription from a micronuclear sequence (STEIN-GAVENS, WELLS and KARRER 1987), other evidence suggests that the micronucleus is generally transcriptionally silent (GOROVSKY 1973, 1980). The somatic macronucleus is transcriptionally active and responsible for the phenotype of the cell. Thus, changes in the micronuclear genome are expected to remain phenotypically silent during vegetative growth of the cells. Tel-1 elements are middle repetitive in the T. thermophila micronuclear genome. Their structure was deduced from studies of blocks of non-telomeric micronuclear epeated C4A2 sequences, (mic C4A2) (CHERRY and BLACKBURN 1985). In all cloned examples, the mic C4A2 sequence was adjacent to a 30-bp conserved sequence. One clone, pl7D59, contained two blocks of mic C4A2 and their adjacent 30-bp conserved sequences in inverted orientation 4.3 kb apart (Figure 1). Tel-1 elements were postulated to be transposons based on their structural similarity to middle repetitive DNA elements with inverted repeats at their ends found in other species; for example, P elements of Drosophila melanogaster and Tcl of Caenorhabditis elegans, which are known to be mobile genetic elements or transposons. Within a species, different strains or isolates typically exhibit distinct genomic distributions of transpo58 C. Wyman and E. H. Blackburn'