[Genetic control of the capacity of Sh. flexneri to exert a lethal effect on macrophages. II. Mapping of the cyt-determinant on the Shigella chromosome].

Crossing experiments showed independence of the genetic determinants controlling the capacity of Sh. flexneri to synthesize the primary S-specific side chains (antigen 3,4) and to produce a lethal action on macrophages cultivated in vitro. Cytotoxicity was restored only in transmission to the R-strain of shigellae of the capacity to synthesize the antigenic factor 3,4 from the cyt+, but not from the cyt-- donor of Sh. flexneri. The determinant responsible for the synthesis of cytotoxin designated as cyt was mapped on the chromosome of shigellae near the rfb gene, controlling the synthesis of the group-specific factor 3,4. The rate of linkage of the cyt+ a 3,4+ was equal to 24.4%. Transductants of the his--cyt-- strain of Sh. flexneri of the S-chemotype acquired the capacity to produce a lethal action on the macrophages with the frequency of the contransduction his+cyt+ equal to 2%. Since the rough (his+R) hybrids of Sh. flexneri and the lysozyme spheroplasts obtained from the cytotoxic strain lost the cytotoxicity whereas the synthesis of the group-specific factor 3,4 provided by itself no lethal effect of the dysentery bacilli on the macrophages it could be supposed that cytotoxin represented an additional thermolabile (in connection with the sensitivity to the temperature action) part of the Sh. flexneri O-antigen.