The extrachromosomal replication of Dictyostelium plasmid Ddp2 requires a cis-acting element and a plasmid-encoded trans-acting factor

Mechanism of action of the Rep protein from the Dictyostelium Ddp2 plasmid family

The two-hybrid system was used to show that the Rep proteins from three members of the Dictyostelium discoideum Ddp2 plasmid family, Ddp2, Ddp5, and Ddp6, form homomultimers but not heteromultimers when expressed in yeast cells. The results with deletion mutations suggest that multiple regions of the Rep proteins are involved in the multimerization. Electrophoretic mobility shift assays with heterologously expressed and purified Ddp2 Rep protein showed that it is a DNA binding protein. The nucleosomal organization of Ddp2 and Ddp6 in their inverted repeat and promoter regions was investigated. Analysis of mutants derived from the Ddp6 plasmid revealed that its Rep protein is required for nucleosome positioning (i.e., phasing) to occur in the promoter region. On the other hand, nucleosome positioning in the inverted repeat regions of both plasmids is not dependent on Rep protein but on either a feature of the DNA sequence or the binding of cellular factors, perhaps the Dictyostelium origin recognition complex. Rep protein is likely involved in transcription regulation and control of DNA replication, specifically amplification of plasmid at low copy numbers. The formation of homomultimers may be required for their regulatory activity.

Dgp1 and Dfp1 are closely related plasmids in the Dictyostelium Ddp2 plasmid family

Dictyostelium plasmids Dgp1 and Dfp1, two members of the Ddp2 plasmid family, are 86% identical in nucleotide sequence. These small (4481 and 5015 bp), high copy number, nuclear plasmids carry both a gene homologous to the Ddp2 rep gene and a long 0.47- to 0. 48-kb inverted repeat region. Their Rep proteins are 82.8% identical in amino acid sequence and carry all 10 of the conserved peptide sequence motifs found in the Ddp2 family Rep proteins. Unlike other members of this family, Dgp1 carries two copies and Dfp1 carries four copies of a 162- to 166-bp direct repeat element. Both the direct and inverted repeat elements, as well as the promoter of the rep gene, are highly conserved (81 to 90% identical) between Dgp1 and Dfp1. In contrast, these regions are not highly conserved and the Rep proteins are only about 40% identical among the other known members of the plasmid family.

Antisense RNA inactivation of gene expression of a cell-cell adhesion protein (gp64) in the cellular slime mold Polysphondylium pallidum

The gp64 protein of Polysphondylium pallidum has been shown to mediate EDTA-stable cell-cell adhesion. To explore the functional role of gp64, we made an antisense RNA expression construct designed to prevent the gene expression of gp64; the construct was introduced into P. pallidum cells and the transformants were characterised. The antisense RNA-expressing clone L3mc2 which had just been harvested at the growth phase tended to re-form in aggregates smaller in size than did the parental cells in either the presence or absence of 10 mM EDTA. In contrast, 6.5-hour starved L3mc2 cells remained considerably dissociated from each other after 5 minutes gyrating, although aggregation gradually increased by 50% during a further 55 minutes gyrating in the presence of 10 mM EDTA. Correspondingly, L3mc2 lacked specifically the cell-cell adhesion protein, gp64. We therefore conclude that the gp64 protein is involved in forming the EDTA-resistant cell-cell contact. In spite of the absence of gp64, L3mc2 exhibited normal developmental processes, a fact which demonstrates that another cell-cell adhesion system exists in the development of Polysphondylium. This is the first report in which an antisense RNA technique was successfully applied to Polysphondylium.

Recombinant glycoprotein production in the slime mould Dictyostelium discoideum

Dictyostelium discoideum is a well known amoeboid organism, with unicellular and multicellular life-cycle stages, that is used for studying cell and developmental biology. With advances in gene-disruption technology and transformation of this organism, many homologous proteins have been expressed either to complement defective proteins or to study basic cell biology. Now, D. discoideum is being used to express heterologous proteins that are difficult to study in other systems, and its unique cell biology is being exploited to facilitate a wide range of protein modifications. In the past year, substantial progress has been made in expressing correctly folded forms of malarial circumsporozoite antigen and rotavirus surface glycoprotein VP7. Exciting developments have also been made in expressing human muscarinic receptors.

Cloning vectors for the production of proteins in Dictyostelium discoideum

We constructed and tested a series of cloning vectors designed to facilitate protein production and purification in Dictyostelium discoideum (Dd). These vectors carry the origin of replication of the Dd high-copy-number plasmid Ddp2, expression cassettes consisting of the strong, constitutive actin (act15) or the inducible discoidin (disI gamma) promoters, a translational start codon upstream from a multiple cloning site and sequences for the addition of epitope or affinity tags at the N- or C-termini of any protein. The affinity tag used corresponds to 7 (N-terminal fusion) or 8 (C-terminal fusion) His residues. The epitope tags correspond to an 11-amino-acid sequence from human c-myc, recognised by monoclonal antibody (mAb) 9E10, and the Glu-Glu-Phe sequence recognised by mAb YL1/2 to alpha-tubulin. Both these mAb are commercially available. The YL1/2 epitope offers a second affinity tag for the purification of proteins under native conditions. The functional competence of the vectors was tested by determining their ability to promote the expression of various Dd myosin constructs. High synthesis levels were obtained for each vector; up to 1 mg of homogenous, functional protein per g of cells was obtained after purification of the recombinant products.

The replication origin position and its relationship to a negative trans-acting transcription regulator encoded by Dictyostelium discoideum nuclear plasmid Ddp1

The replication origin of the Dictyostelium discoideum plasmid Ddp1 was localized to a 543-bp region. This includes most of the AT-rich intergenic region between the G1 and G5/D6 genes containing both of their promoters and multiple copies of a TTTTGACT repeat. The G5/D6 gene, which lies adjacent to, and partially overlaps, the 543-bp origin region, encodes a trans-acting factor that negatively regulates transcription of the G4/D5 gene. Inactivation of the G5/D6 gene led to expression of a transcript (G6) 0.2 kb larger than the D5 transcript from the G4/D5 gene in vegetative and developing cells. The G5/D6 gene also regulates transcription of the G1, G2/G3/D4 and G5/D6 genes either alone or in concert with other Ddp1 gene products.

An autonomously propagating luciferase-encoding vector for Dictyostelium discoideum

We have constructed a luc reporter vector for Dictyostelium discoideum using a 626-bp fragment from the nuclear-associated plasmid Ddp2. The ori from Ddp2 is localized within this fragment and was used to provide an autonomous replication sequence for the reporter vector. This reporter vector was stably retained in D. discoideum AX3K cells without alteration. The vector molecule was also found to exist in relatively low copy number compared to other Dictyostelium vectors in the transformed cells. We demonstrated the utility of this vector as a reporter vector with glycogen synthase promoter/luc fusions of varying sizes.

Production and secretion of recombinant proteins in Dictyostelium discoideum

We have expressed useful amounts of three recombinant proteins in a new eukaryotic host/vector system. The cellular slime mold Dictyostelium discoideum efficiently secreted two recombinant products, a soluble form of the normally cell surface associated D. discoideum glycoprotein (PsA) and the heterologous protein glutathione-S-transferase (GST) from Schistosoma japonicum, while the enzyme beta-glucuronidase (GUS) from Escherichia coli was cell associated. Up to 20mg/l of recombinant PsA and 1mg/l of GST were obtained after purification from a standard, peptone based growth medium. The secretion signal peptide was correctly cleaved from the recombinant GST- and PsA-proteins and the expression of recombinant PsA was shown to be stable for at least one hundred generations in the absence of selection.

Selection of Dictyostelium discoideum transformants and analysis of vector maintenance using live bacteria resistant to G418

A protocol that allows the rapid isolation and growth of large numbers of independent G418-resistant Dictyostelium discoideum transformant colonies on the surface of agar media with live bacteria was developed. Transformants grown under these conditions form normal fruiting bodies. Discovery that aggregation of nontransformants was inhibited at a nonselective level of G418 (25 to 35 micrograms/ml) led to the development of a vector maintenance assay. Using this assay we examined the stability of recombinant plasmids derived from the D. discoideum native plasmids Ddp1 and Ddp2. We conclude that the origin of replication of plasmid Ddp1 does not alone confer stable maintenance and thus, Ddp1 must bear additional sequences required for its own maintenance. Analysis of the maintenance of vectors derived from Ddp2 showed that autonomously replicating shuttle vectors that contained bacterial plasmid DNA and from which one element of the Ddp2 inverted repeat was removed were much less stable than vectors that contained a complete inverted repeat or that did not carry a bacterial plasmid. Sequences between the 3' end of the rep gene and the inverted repeat appear to play a role in plasmid maintenance. An intact rep gene and one copy of the inverted repeat element were required for extrachromosomal replication. Maintenance of extrachromosomal vectors was found to be strain dependent. Four traits distinguishing integrating vectors from those capable of autonomous replication were identified.

Dictyostelium giganteum plasmid Dgp1 is a member of the Ddp2 plasmid family

Dgp1, a circular 4.4-kb plasmid found in the nuclei of Dictyostelium giganteum strain DG61, is a member of the same plasmid family as plasmids Ddp2 and pDG1. Dgp1 has sequence similarity to a conserved region of the Ddp2 and pDG1 open reading frames. As with Ddp2 and pDG1, a single large RNA is transcribed from Dgp1. This 3.3-kb transcript is present at about 350 copies per vegetative cell. The transcript abundance decreased about 10-fold in early aggregation and continued at this lower level until late culmination when it returned to the level seen in vegetative cells. Dgp1 has a repeat of several hundred base pairs in a location, relative to the transcribed region, similar to the inverted repeats found in Ddp2 and pDG1. Dgp1 cannot be maintained as a plasmid in Dictyostelium discoideum AX4 cells, suggesting that Dgp1 carries species-specific maintenance elements.

The ble gene of Streptoalloteichus hindustanus as a new selectable marker for Dictyostelium discoideum confers resistance to phleomycin

An expression cassette has been constructed which allows expression of the ble gene isolated from Streptoalloteichus hindustanus in Dictyostelium discoideum. This construct has been shown to confer resistance to the bleomycin related antibiotic phleomycin. since the uptake of phleomycin by the cells is pH dependent, we established conditions that allow selection of phleomycin-resistant transformants. Vectors pfeI and pfeII contain, in addition to the cassette, a 592 bp fragment of the D. discoideum plasmid Ddp2 that enables the plasmids to replicate extrachromosomally in Dictyostelium when transformed into a strain that expresses a Ddp2-specific transacting factor (12). pfeI and pfeII contain various unique restriction enzyme sites for cloning. They differ in the G/C-content of the sequence upstream of the ATG start codon.