Regulation of the fdhF gene encoding the selenopolypeptide for benzyl viologen-linked formate dehydrogenase in Escherichia coli

Summary In Podospora anserina a chromosome walk near the mating type locus was made possible through isolation of genomic sequences linked to a plasmid integrated in this part of the genome. Genetic analysis of 86 transformants obtained from the 5 first cosmids of this walk was performed. These data and those reported elsewhere for cosmids resulting from another chromosome walk allow us to draw two clear-cut rules for transformation with cosmids. First, the large majority of transformants arise from integration at the resident locus, contrasting with the heterologous process which predominates for plasmids. Second, all homologous integrations are highly unstable while all non-homologous integrations are stable. Analysis of the timing of the instability reveals that loss of the selective marker is probably limited to the fruiting body.