60 MOLECULES, VIRUSES, AND BACTERIA 



( Ephrussi-Taylor, 1958). Thus recombination seems to be a conse- 

 quence of growth and the synthesis of a new chromosome in the pres- 

 ence of the acquired DNA molecule. 



Experiments have also indicated that some kind of pairing must 

 occur between the acquired molecule and the recipient cell's genome 

 (Schaeffer, 1958). 



Only one detailed study of recombination in transformation has 

 been performed (Lacks and Hotchkiss, 1960). In Pneumococcus, eight 

 mutations were obtained for the loss of the ability to synthesize amylo- 

 maltase, an enzyme permitting the cell to grow on maltose. Since other 

 factors besides recombination frequency play a part in determining a 

 particular transformation frequency in a given experiment, a method is 

 required to correct for these irrelevant influences. This consisted in 

 marking each donor DNA not only with a particular configuration of 

 the maltose locus, but also with an unlinked marker, streptomycin re- 

 sistance. Recombination frequency is expressed as a ratio of amylomal- 

 tase-positive cells to streptomycin-resistant cells, and for any given 

 type of recombination this ratio is reasonably constant. 



Each mutant treated with wild-type DNA is found to have a char- 

 acteristic specific probability of being transformed back to maltase 

 plus, as can be seen from the first column of Table I. Further, cross 



TABLE I 



Recombination in the amylomaltase locus of pneumococcus (after Lacks 

 and Hotchkiss, 1960). Values are for the ratio of amylomaltase-positive to strepto- 

 mycin-resistant cells. Standard deviations have been published only for transforma- 

 tion of mutants by wild-type DNA. They are, for the mutants in alphabetical order, 

 0.18, 0.10, 0.03, 0.28, 0.20, 0.33, 0.05, and 0.22. 



testing was performed in order to construct a map of the mutants. 

 That is, each mutant was used as a receiver and also, after it was 

 marked with streptomycin resistance, as a donor with respect to every 



