134 rHARLKS A. THOMAS, ,IH. 



Kaisrr and llogncs.s purilicd the transducing pha^c hy making use 

 of a property of hunbda-dg, namely, that it has a (HfTcrcnt density 

 than ordinary hinil)da. The DNA was then extracted fioni these phage 

 by the phenol technique, and the resulting DNA could be shown to 

 produce Gal"" colonies when added to appropriately lysogenized Gal" 

 cells in the presence of "helper" phage. While the details of this cxpci-i- 

 ment are somewhat technical, the net result is a transfontiation of the 

 recipient cell to Gal^ which is scored on EMB-galactose plates. A 

 further examination of these cells has revealed that they are Gal"" 

 because they have become lysogenic for lambda-dg. If this is true, these 

 transformants should also contain other genes (outside the dg region) 

 as well. This was tested by looking for the cotransfer of r and mi genes 

 which are located at some distance from the end of the dg region. It was 

 found that bacteria which had received the Gal"^ also had a high proba- 

 bility of receiving both the c and mi genes as well. This suggested 

 strongly that the molecule which was responsible for the transformation 

 contained the entire lambda-dg chromosome. 



3. Transformation with DNA frorn Ordinary Lambda 



These studies on lambda DNA were continued by Kaiser (1962), 

 who developed an assay for ordinaiy lambda DNA; in this case the 

 assay was based on the production of infectious phage particles rather 

 than the transformation of a recipient cell. Again the role of "helpei'" 

 phage is important. A non-lysogenic strain of E. coli K12 was infected 

 with helper lambda (bearing the i^-'* marker) at a multiplicity of 10. 

 After a suitable period for adsorption, the phage-infected bacteria were 

 washed and then mixed with the diluted DNA solution. This DNA was 

 prepared by phenol extraction of a purified preparation of lambda phage 

 bearing the appropriate genetic markers. After a period of ap])i()xiniately 

 40 minutes DNase was added to digest any extracelhilai- DXA and 

 appropriate dilutions of this mixture were plated on indicator bacteria 

 to characterize the progeny phage genetically. These indicator strains 

 will not record jihage which are produced by the "helper" jihage, but 

 they will record phage bearing one or more genetic markers furnished by 

 the DNA. The map of the lambda phage from which the DN.\ was 

 prepared is given in Fig. 8. It was found that when the DNA contrii)uted 

 genes, it contributed all of the markers on the above map. There was the 

 apparent simultaneous transformation of all of the markers, and in this 

 regard it was similar to phage infection. Thus the infecting DNA 

 molecule comprises the entire chromosome of the phage. When this DNA 

 preparation is stirred, the molecules are broken near the middle, causing 

 them to sediment more slowly in a sucrose density gradient. The ratio 



