A genetic system to detect mitotic recombination between repeated chromosomal sequences in Drosophila Schneider line 2 cells

Single-cell cloning enables the selection of more productive Drosophila melanogaster S2 cells for recombinant protein expression

The generation of monoclonal cell lines is an important early process development step for recombinant protein production. Although single-cell cloning is an established method in mammalian cell lines, straightforward protocols are not yet available for insect cells. We describe a new method for the generation of monoclonal insect cells without using fetal bovine serum and/or feeder cells pretreated by irradiation or exposure to mitomycin. Highly productive clones of Drosophila melanogaster S2 cells were prepared in a two-step procedure, comprising the establishment of a polyclonal population and subsequent single cell isolation by limiting dilution. Necessary growth factors were provided by co-cultivation of single transformants with untransfected feeder cells, which were later removed by antibiotic selection. Enhanced expression of EGFP and two target peptides was confirmed by flow cytometry and dot/western blotting. Highly productive clones were stable, showed a uniform expression profile and typically a sixfold to tenfold increase in cell-specific productivity.

Analysis of extrachromosomal homologous recombination in cultured silkworm cells

Double-strand breaks (DSBs) are potentially lethal lesions causing the loss of chromosomal information. Eukaryotic cells have evolved the error-free repair systems of DSBs by homologous recombination (HR) through gene conversion with or without crossing over. In this study, we have developed a rapid assay system for extrachromosomal HR events in the cultured silkworm BmN4 cells. When HR occurs within the disrupted luciferase gene, an enzymatically active luciferase is restored and expressed. Our results strongly suggest that error-prone single strand annealing (SSA) accounts for the majority of extrachromosomal recombination processes in the cells. However, upon the substrates which cannot be repaired through SSA, DSBs were efficiently repaired though gene conversion. The rapid and sensitive HR assay system developed in the present study is expected to be a powerful tool for the identification and analysis of HR-related genes in the silkworm.