III. BACTERIOPHAGE DNA AND BACTERIAL DNA 133 



circular map. In the exploration of the genetic structure of any organism 

 one is restricted to that region of the map which contains recognizable 

 markers. The available markers in lambda are in a region which is 

 only about 20 units long, and there is no evidence of circularity. 



The discovery of the circular map in T2 and T4 makes it quite 

 certain that additional map segments will not be added as additional 

 markers are discovered. Thus one can state with confidence that the 

 map in T2 is known, unitary, and circular. The DNA molecule liberated 

 from the phage, containing essentially all of the nucleotides in the virus, 

 is unitaiy, but almost surely linear. Thus, at present, one would like to 

 know the relationship between the circular map and the linear molecule. 

 Perhaps each DNA molecule has genes in the same order but the 

 beginning and end points are different. Such a circularly permuted 

 molecule would make the genetic results easily understood. Alterna- 

 tively, the molecule and map could be unique but two (or an even 

 number of) cross-overs always occur during a recombinational event 

 (Stahl, 1962). Some of these possibilities can be tested. At the present 

 time, one is limited by the fact that no sensitive transformation assay 

 for T2 segments has been developed. 



2. Transformation by Lamhda-dg DNA 



In the case of phage lambda the situation is more favorable as a 

 result of the work of Kaiser and Hogness (1960) and Kaiser (1962) 

 who have demonstrated a transformation system with lambda-dg, and 

 lambda DNA. When a bacterial strain which is lysogenic for lambda 

 is induced by ultraviolet light, the vegetative phase of growth is initiated 

 and new phage particles are synthesized and subsequently released. 

 Approximately one in 10" of these new particles is lambda-dg. These 

 phage are different from ordinary lambda in a number of ways. This 

 phage particle can transduce the galactose genes from the bacteria in 

 which it was grown to the bacteria it infects. It is defective in the sense 

 that it is missing a sizable fraction of its genetic map (Arber, 1958) 

 and it has a veiy small chance of establishing the lysogenic condition 

 compared wth normal lambda. Independently arising lambda-dg phage 

 can be shown to differ in their density when banded in CsCl, indicating 

 that they have a variable DNA content (Weigle, Meselson, and Paigen, 

 1959). It is currently thought that these lambda-dg phage are the result 

 of a substitution of bacterial genes for some of the phage genes. This 

 exchange appears to be confined to a special region of the lambda map, 

 and appears to be unequal in the sense that the resulting lambda-dg 

 may have somewhat more (or somewhat less) total DNA than ordinary 

 lambda. 



