Studies on the mechanism of intracistronic recombination in partillay diploid strains of Escherichia coli

The central phenomenon of genetics is recombination. It is through this phenomenon that genetic activity can be analyzed. Recombination of linked genetic factors can be defined as an interaction between homologous DNA molecules to yield DKA molecules having a nucleotide sequence partly that of one parental molecule and partly that of the other. It is amazing, therefore, to note that the mechanism of recombination, the steps leading to and involved in the formation of the recombinant molecule, remains one of the few phenomena which has not been revealed by modern molecular biology. It is with the hope of shedding some light upon the mechanism of recombination that this study has been undertaken. Single, isolated recombinational events have been studied via a technique of positionally fixing a small number of histidineless bacterial cells upon a membrane filter. A recombinant event within the histidine operon is expressed as a histidine plus colony. The bacterial strain studied is an E. coli K12 F', diploid in the histidine region. The type of mutations involved in the recombinational event are trans-intracistronic mutations, that is non-complimenting transmutations within a cistron, a nucleotide sequence coding for the synthesis of a single polypeptide chain. Analyses of intracistronic recombinational events in the D and the A cistron of the histidine operon yield data which suggest a model for the mechanism of intracistronic recombination. This model assumes that the homologous region of the F-merogenote, in this case the histidine operon, lies in juxtaposition to the histidine operon of the chromosome and that the two replicate and/or transcribe in unison. The recombinational event is envisioned as the reciprocal exchange of single strands usually com mencing at the operator (G) end (probably because of uniform single-stranded breaks occurring as a function of transcription or replication) and terminating between the two mutations. In some cases, the event may initiate between the two mutations and proceed throughout the length of the operon. Thus two heteroduplex molecules are produced having one parental strand and one recombinant strand. After the simultaneous semi-conservative replication of the two heteroduplex molecules, two F-merogenotes, one parental and one recombinant, and two endogenotes, one parental and one recombinant are produced. This mechanism frequently involves a "monitoring out" of minus information. If the base miss-pairings in the newly formed heteroduplex region caused molecular distortions comparable to UV-induced thymine dimers then a repair-monitoring type of repair system might remove a segment of the strand including the mutation, use the remaining complementary wild type strand as template and replace the missing segment with the wild type complementary sequence. The extent of monitoring out is related to the nature of the mutation involved. During the course of this study, the relative map distances of the four D mutations involved was determined relative to D[lowered 1], through spontaneous rate recombination studies. The position of two of the D mutations and two of the A mutations relative to the operator was also determined.Biology and Biochemistry, Department o