A novel, negative selectable marker for gene disruption in Dictyostelium

Transformation of Dictyostelium discoideum with plasmid DNA

DNA-mediated transformation is one of the most widely used techniques to study gene function. The eukaryote Dictyostelium discoideum is amenable to numerous genetic manipulations that require insertion of foreign DNA into cells. Here we describe two commonly used methods to transform Dictyostelium cells: calcium phosphate precipitation, resulting in high copy number transformants; and electroporation, an effective technique for producing single integration events into genomic DNA. Single integrations are required for gene disruption by homologous recombination. We also discuss how different selection markers affect vector copy number in transformants and explain why blasticidin has become the preferred selectable marker for making gene knockouts. Both procedures can be accomplished in less than 2 h of hands-on time; however, the calcium phosphate precipitation method contains several incubations, including one of at least 4 h, so the total time required for the transformation is approximately 8 h.

Gene targeting in mosquito cells: a demonstration of 'knockout' technology in extrachromosomal gene arrays

BACKGROUND

Gene targeting would offer a number of advantages over current transposon-based strategies for insect transformation. These include freedom from both position effects associated with quasi-random integration and concerns over transgene instability mediated by endogenous transposases, independence from phylogenetic restrictions on transposon mobility and the ability to generate gene knockouts.

RESULTS

We describe here our initial investigations of gene targeting in the mosquito. The target site was a hygromycin resistance gene, stably maintained as part of an extrachromosomal array. Using a promoter-trap strategy to enrich for targeted events, a neomycin resistance gene was integrated into the target site. This resulted in knockout of hygromycin resistance concurrent with the expression of high levels of neomycin resistance from the resident promoter. PCR amplification of the targeted site generated a product that was specific to the targeted cell line and consistent with precise integration of the neomycin resistance gene into the 5' end of the hygromycin resistance gene. Sequencing of the PCR product and Southern analysis of cellular DNA subsequently confirmed this molecular structure.

CONCLUSIONS

These experiments provide the first demonstration of gene targeting in mosquito tissue and show that mosquito cells possess the necessary machinery to bring about precise integration of exogenous sequences through homologous recombination. Further development of these procedures and their extension to chromosomally located targets hold much promise for the exploitation of gene targeting in a wide range of medically and economically important insect species.

Dictyostelium as model system for studies of the actin cytoskeleton by molecular genetics

The actin cytoskeleton is an essential structure for most movements at the cellular and intracellular level. Whereas for contraction a muscle cell requires a rather static organisation of cytoskeletal proteins, cell motility of amoeboid cells relies on a tremendously dynamic turnover of filamentous networks in a matter of seconds and at distinct regions inside the cell. The best model system for studying cell motility is Dictyostelium discoideum. The cells live as single amoebae but can also start a developmental program that leads to multicellular stages and differentiation into simple types of tissues. Thus, cell motility can be studied on single cells and on cells in a tissue-like aggregate. The ability to combine protein purification and biochemistry with fairly easy molecular genetics is a unique feature for investigation of the cytoskeleton and cell motility. The actin cytoskeleton in Dictyostelium harbours essentially all classes of actin-binding proteins that have been found throughout eukaryotes. By conventional mutagenesis, gene disruption, antisense approaches, or gene replacements many genes that code for cytoskeletal proteins have been disrupted, and altered phenotypes in transformants that lacked one or more of those cytoskeletal proteins allowed solid conclusions about their in vivo function. In addition, tagging the proteins or selected domains with green fluorescent protein allows the monitoring of protein redistribution during cell movement. Gene tagging by restriction enzyme mediated integration of vectors and the ongoing international genome and cDNA sequencing projects offer the chance to understand the dynamics of the cytoskeleton by identification and functional characterisation of all proteins involved.

Blocking the ends of transforming DNA enhances gene targeting in Dictyostelium

Eukaryotic gene targeting by means of gene replacement vectors is often complicated by unwanted plasmid insertion events involving the ends of transforming DNA molecules. These undesirable and often multiple insertions occur both randomly (i.e. non-homologously) and at the targeted locus. By blocking the 3' ends of transforming DNA with 2'3' dideoxynucleotides, we have reduced the frequency of end-mediated DNA insertion in Dictyostelium amoebae. As a result, only one copy of the selectable gene is introduced at the target locus to achieve a precise gene disruption.