Summary: New types of donor strains have been obtained by indirect selection froman IF strain bearing the autonomous plasmid SCP 1. The new donors differ from the previously known type of donor (NF) in being, usually, unstable, segregating numerous IF and UF variants, and in their pattern of donation to UF strains (recipient strains lacking SCP 1). Whereas the chromosome fragments donated by NF strains have neither end at a constant position, those transferred by the new donors have one constant and one variable end. The new donors are designated according to the marker donated with the highest frequency - that is, nearest to the constant end of the chromosome fragments. It is postulated that they arise by interaction between SCP 1 and the chromosome, perhaps to produce substituted plasmids analogous to F prime strains of , which may mobilize the chromosome by donor crossing-over. Most of the new donors had a white colony phenotype, although producing apparently normal spores.


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