A linear shuttle vector for yeast and the hypotrichous ciliate Stylonychia

Mapping the germ-line and somatic genomes of a ciliated protozoan, Tetrahymena thermophila

Ciliates are among the very few eukaryotes in which the powers of molecular biology, conventional genetics, and microbial methodology can be synergistically combined. Because ciliates also are distant relatives of vertebrates, fungi, and plants, the sequencing of a ciliate genome will be of import to our understanding of eukaryotic biology. Tetrahymena thermophila is the only ciliate in which a systematic genetic mapping of DNA polymorphisms has begun. Tetrahymena has many biological features that make it a specially or uniquely useful experimental system for fundamental research in cell and molecular biology and for biotechnological applications. A key factor in the usefulness of Tetrahymena is the speed, facility, and versatility with which it can be cultivated under a wide range of nutrient conditions, temperature, and scale. This article describes the progress made in genetically and physically mapping the genomes of T. thermophila: the micronuclear (germ-line) genome, which is not transcriptionally expressed, and the macronuclear (somatic) fragmented genome, which is actively expressed and determines the cell's phenotype.

Replication of circular and linear SV40-based plasmids in monkey cells

Three plasmids were derived from a common SV40-based parent. A circular plasmid (pYACneoC) contained the SV40 ori and two sets of 50 bp of human telomeric sequences. By differential enzyme digestion, two linear plasmids were generated from the circular form, one (pYACneoL) terminating with, and the other (pYACneoN) free of telomeric sequences. The replicative features of the circular and of both linear plasmids were assayed by transfecting COS-7 cells. At various times after transfection, the low-molecular-weight DNA was extracted, and the fraction of molecules that had replicated was determined by Dpnl digestion. We demonstrate that about half of the circular plasmid molecules replicate, but only during a short time interval immediately following transfection. No replication was observed in the case of the two linear plasmids. However, the function of the SV40 origin is potentially present in the molecules, since circular forms that do replicate can be recovered from both linear plasmids. The extent of replication of circularized pYACneoL is similar to that of pYACneoC, whereas a lower fraction of circularized pYACneoN molecules replicate. These results are discussed in terms of the possible influence of the DNA structure on the viral ori, and of the influence of the host cell functions on viral replication.

Efficient transformation of cam2, a behavioral mutant of Paramecium tetraurelia, with the calmodulin gene

An Ile-136----Thr substitution in calmodulin reduces the Ca(2+)-dependent K+ currents of cam2, a behavioral mutant of Paramecium tetraurelia, and renders it overly susceptible to BaCl2. DNA fragments carrying the wild-type CAM gene injected into cam2 macronuclei reverted these phenotypes in the clonal descendants of the recipients. Tetrahymena telomeric sequences, added in vitro to the fragment termini before injection, enhanced the efficiency and quality of transformation. Five times 10(4) copies of such fragments consistently restored the phenotypes to near normal; even 10(3) or fewer copies could still effect weak transformation. The restored phenotypes were stable for greater than 20 fissions in many clones and were lost after autogamy. We examined the fate of the injected fragments in the transformed clones and discuss the possible application of this efficient transformation in the cloning of other genes of P. tetraurelia.

A human DNA telomeric fragment contains yeast ARS and mitotic stabilizing sequences

A minilibrary of human DNA fragments was prepared in the vector YIP5 from a DNA preparation enriched for telomeric sequences. Screening of the library produced one clone that hybridized to the TTAGGG sequence. The cloned DNA fragment was shown to be telomeric by a number of criteria. In situ hybridization to metaphase human chromosomes showed that the fragment hybridized to the tips of all human chromosomes. The fragment contained at least two yeast autonomously replicating sequences (ARS) and stabilizing sequences, since it transformed Saccharomyces cerevisiae with high efficiency, giving rise to clones which were mitotically stable under non-selective growth.

Functional telomere formation in yeast using synthetic C4A2 sequences

A yeast artificial chromosome (YAC) was constructed with a native autonomous replicating sequence (ARS) flanked telomere at one end and a 50-bp synthetic oligonucleotide of C4A2 repeats at the other. This was done in order to determine whether the presence of the flanking ARS sequence is required for telomere function. This construct was introduced into two different yeast strains: one mutated in the recombination function RAD52 and the other wild type for this gene. Both strains gave rise to autonomously replicating artificial chromosomes. The molecules in the RAD52 strain were rearranged dimers terminating at both ends with Tetrahymena telomeres, whereas in the rad52 strain two classes of YACs were found: rearranged dimers and elements bearing an ARS-free telomere. The presence of the latter class of molecules confirmed the finding of Wellinger and Zakian (1989, Proc. Natl. Acad. Sci. USA 86, 973-977) that the flanking ARS is not required for telomere function. Furthermore, in this class of molecules the ARS-free telomeric end was shortened as a result of deletions that removed some distal pBR322 sequences and some C4A2 repeats. The size of the resulting YACs ranged from 7.7 to 9 kb, considerably below the size threshold found by Zakian et al. (1986, Mol. Cell. Biol. 6, 925-932) for CEN4 artificial plasmids. An explanation for the structural instability of the ARS-free end of the YACs is suggested.

Transformation of Paramecium by microinjection of a cloned serotype gene

Paramecia of a given serotype express only one of several possible surface proteins called immobilization antigens (i-antigens). A 16-kilobase plasmid containing the gene for immobilization antigen A from Paramecium tetraurelia, stock 51, was injected into the macronucleus of deletion mutant d12, which lacks that gene. Approximately 40% of the injected cells acquired the ability to express serotype A at 34 degrees C. Expression appeared to be regulated normally. The transformed cells, like wild type, could be switched to serotype B by antiserum treatment and culture at 19 degrees C; on transfer to 34 degrees C, they switched back to serotype A expression. Many of the lines retained the ability to express serotype A until autogamy, when the old macronucleus is replaced by a new one derived from the micronucleus. DNA from transformants contained the injected plasmid sequences, which were replicated within the paramecia. No evidence for integration was obtained. The majority of replicated plasmid DNA comigrated with a linearized form of the input plasmid. Nonetheless, the pattern of restriction fragments generated by transformant DNA and that generated by input plasmid DNA are identical and consistent with a circular rather than a linear map. These conflicting observations can be reconciled by assuming that a mixture of different linear fragments is present in the transformants, each derived from the circular plasmid by breakage at a different point. Copy-number determinations suggest the presence of 45,000-135,000 copies of the injected plasmid per transformed cell. These results suggest that the injected DNA contains information sufficient for both controlled expression and autonomous replication in Paramecium.