Zinder and Lederberg (Nobel 1958) placed two auxotrophic strains of Salmonella on opposite sides of a Davis U-tube.

Prototrophs were recovered from the side containing LA-22 cells, but not from the side containing LA-2 cells.

Adding DNase, an enzyme that digests DNA, did not prevent recombination, so this is not transformation.

The filter prevented cell contact, so this is not conjugation.

• Recombination stopped when the pore size of the Davis U-tube was reduced below the size of phages.

  • This transduction was mediated by phage P22, starting as a prophage in LA-22 cells. Rarely, some P22 enter the lytic phase, exit the LA-22 cells, and cross the filter to infect LA-2 cells. When LA-2 cells are lysed, some LA-2 DNA may be packaged in P22 heads, which can move back across the filter to infect LA-22 cells, enter lysogeny and producing recombinants.