Section 3 — Molecular and Microbial Genetics 



ase. Dilution of the preparations leads to an 

 approximately exponential decrease of infecti- 

 vity. Two alternative explanations are suggested: 

 transfer of genetic information of the virus by 

 means of RNA synthetized by infected host 

 cells, or formation in these cells of an infectious 

 RNA-DNA complex. Resistance of the infective 

 principle to DN-ase and its susceptibility to RN- 

 ase seem to speak in favour of the first explana- 

 tion. 



3.37. DNA Transformation for Oxytetracycline 

 Resistance in B. subtilis. M.Polsinelli (Pavia, 

 Italy). 



A strain of B.subtilis resistant to about 300y/ml 

 of oxytetracycline has been produced through 

 multistep selection and the DNA prepared from 

 it used for transforming a sensitive strain. 



Transformation was carried out with such 

 DNA and selection of transformed bacteria 

 performed on media containing a minimal 

 inhibiting concentration of oxytetracycline. 

 The degree of resistance of transformed clones 

 was individually tested. The population of the 

 transformed bacteria shows a trimodal distri- 

 bution of resistance suggesting that at least two 

 genes are involved in the process. 



Transformation by means of DNA taken from 

 clones belonging to the first and second peak 

 of the distribution shows that such clones are 

 transmitting a single genetic factor for oxy- 

 tetracycline resistance, while DNA from clones 

 of the third peak transforms sensitive bacteria 

 in a way similar to that of the original resistant 

 strain. Further results will also be discussed. 



3.38. Multi-site Mutations and Deletions. Minna B. 

 Rotheim and Arnold W. Ravin (New York, 

 U.S.A.). 



By means of DNA-mediated transformations 

 in pneumococcus 13 spontaneous mutations to 

 streptomycin resistance have been mapped at 

 only three sites, which may be designated A, B 

 and C. Mutations occurring at she C replace, 

 but never combine with, those at sites A and B. 

 For this reason mutations at site C are considered 

 to be multisite mutations; such mutations have 

 generally been considered to be deletions. Marker 

 str-r41 has been mapped at site C. In one 

 culture of str-r41 a new mutant appeared having 

 a hundred-fold lower level of resistance. Experi- 

 mental data indicate that the lower level of 

 resistance is not due to the action of a suppressor 

 on a still present str-r41 mutation, but due to 



a new mutation. This latter mutation has been 

 called str-r42, and in transformation reactions 

 may be replaced by, but never combines with, 

 marker str-r41. Moreover, by recombining with 

 site A and B mutations, the str-r42 mutation is 

 shown to occupy a new site (D). Therefore, mar- 

 ker str-r42 lies within the region covered by 

 the mutation str-r41, suggesting that marker 

 str-41 may partially revert to leave a small 

 mutated segment which confers a lower level of 

 streptomycin resistance. Defining deletions oper- 

 ationally as mutations incapable of back muta- 

 tion within the same segment of genetic material 

 that was involved in the original mutational 

 event, these results establish str-r41 as a multisite 

 mutation that is not a deletion. 



3.39. Transformation of Genetic Traits Associated 

 with Sporulation in Bacillus subtilis. John 

 Spizizen, Bernard Reilly, and B. Dahl 

 (Minneapolis, U.S.A.). 



Studies in Bacillus species strongly indicate an 

 association of a number of enzyme systems 

 formed in the post-logarithmic growth period 

 with the sporulation process. Thus the forma- 

 tion of proteolytic and cell-wall lytic enzymes 

 appears to be associated with sporulation. A 

 genetic approach to this problem has been 

 initiated with the finding of a linkage group con- 

 taining genes which control the synthesis of a 

 proteolytic enzyme, a wall-lytic factor or en- 

 zyme, and sporulation. These studies were 

 made with transformable Bacillus subtilis, 

 strain 168. Asporogenous mutants unable to 

 synthesize protease and the lytic factor could 

 be transformed with deoxyribonucleic acid 

 (DNA) isolated from sporogenous strains with 

 the production of sporogenous organisms con- 

 taining also genes for protease and lytic factor. 



Partially asporogenous mutants have also 

 been found which contain the genes for protease 

 and lytic factor. The asporogenous defect in 

 such mutants appears to have arisen from a 

 separate locus, so that sporogenous recombinants 

 could be obtained with crosses of the two as- 

 porogenous types. Evidence will also be pre- 

 sented which strongly suggests that certain par- 

 tially asporogenous mutants are heterogenotic 

 for some regions of the sporulation genome. 



3.40. Genetic Studies on Sporogenesis of Bacillus 

 subtilis. I. Takahashi (Ottawa, Canada). 



By the use of temperate bacteriophage PBS 1, 



31 



