Genetic Recombination in Dna-induced Transformation

HE science of genetics has undergone a tremendous development as a result T of the discovery of novel systems of exchange of genetic material. The first new system to be discovered was that of DNA-induced bacterial transformation (AVERY, MACLEOD and MCCARTY 1944) , yet as a purely genetic tool this system has been little used, in spite of possessing unique experimental possibilities. The present report, and those which have preceded it ( SICARD 1964; EPHRUSSITAYLOR, SICARD and KAMEN 1965) describe experiments undertaken to establish the procedures of genetic analysis of fine structure by transformation. The general features of transformation as an experimental system are the following: (1) DNA is extracted from donor cells. This extraction results in the breaking down of the bacterial chromosome into a relatively large number of fragments, each of which is large enough to comprise several genes (10,000 to 20,000 base pairs). (2) Competent recipient cells react randomly with large DNA fragments, absorbing one or a few. ( 3 ) An absorbed fragment undergoes genetic recombination with the region of the bacterial chromosome with which it is homologous. Gene segregation is due, therefore, to two distinct processes: (a) the breakdown during DNA extraction of the donor chromosome into fragments, followed by random uptake of these fragments by recipient cells, and (b) segregation, through recombination of regions of a given absorbed fragment with the recipient cell's chromosome, leading to reassortment of donor and recipient genes within the homologous region. The mapping of a bacterial chromosome by transformation involves, therefore, operating at two distinct levels: (a) the realignment of fragments in proper order and (b) the establishment of gene order within fragments. The present paper deals with the second level of analysis. The problem of deciding (when two genes are linked, i.e., located on the same DNA fragment, is readily solved when linkage is strong: the simultaneous acquisition of both genes by a single cell is so frequent that it cannot be due to absorption of two different DNA fragments. When linkage is weak, however, detection of linkage is best performed using a kinetic approach. The curve describing the numbers of double transformants as a function of DNA concentra-