Chromosome transfer has been studied in mating pairs formed during a 5-minute interval, under conditions that prevent the formation of new pairs. It has been found that, despite restriction of the period of effective contact formation, there is a considerable spread in the time at which transfer is initiated in the different mating pairs. In some pairs there is a delay of as much as 15 minutes before transfer commences.The presence of broth in the medium in which transfer takes place leads to a marked reduction in the stability of pairs involving some Hfr strains, but has little or no effect on pairs involving others. The action of broth can be ascribed to the effect of a complete mixture of amino-acids on the metabolic activity of the donor cells.A segment of chromosome which has penetrated from the Hfr into the F−cell may be subsequently withdrawn. Withdrawal may take place in more than half of the mating pairs, and probably occurs at the time when the donor and recipient cells separate. It is prevented if the chromosome breaks when conjugation is interrupted.

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SUMMARY19 independent recombination-deficient mutants were isolated. 7 carried mutations that mapped near or in the recB and recC genes between thyA and argA. IO mutants carried mutations cotransducible with pheA and exhibited no complementation with recA in temporary zygotic diploids.Two new genes controlling recombination have been identified. Strain PC 0297 carried a mutation designated recGz62, and was UV-and X-ray-sensitive. It was located between pyrE and ilvA. Strain PC 125o was only slightly UV-sensitive, was X-ray-resistant and carried the mutation designated recHI66 which was located between pheA and cysQ.

SUMMARYMapping of markers on the Escherichia coli chromosome by the gradient of transmission is presented. The method appeared to be useful and accurate if chromosome withdrawal is prevented and if the viability of the zygotes is assured.The guaC was mapped by this method in the 89-90 min region of the chromosome map of TAYLOR AND TROTTER.

A number of mutants ofE. coliK 12, deficient in purine biosynthesis, have been isolated and the biochemical blocks have been determined. The mutations were mapped in conjugation experiments. In some cases the differences in penetration times were too small to determine the exact order of the loci by interrupted mating experiments. In these cases the recombination frequencies were determined in four factor crosses. In this way the location of eleven different purine markers has been determined. The loci are scattered over the chromosome. Only two groups of linked genes were found.

SUMMARYIn this paper a mathematical analysis based on the physical exchange of genetic material is presented for a four-factor cross. The incorporation frequency of donor markers and the relative map distances may be accurately estimated from the frequencies of the eight recombinant classes. The results obtained in KI2 x KI2 and KI2 × B crosses are in good agreement with the theory.

A relation between linkage frequency of an unselected marker and transfer time based on a physical exchange of genetic material was developed for Escherichia coli crosses. Crosses performed under standardised conditions have shown that the relation was valid. The linkage frequency is determined by two constants and one parameter. The constants are the incorporation frequency for donor fragments and the average number of breakage events per unit length. The variable in the relation is the distance in time units between selected and unselected marker.

A variable delay among mating pairs in the time between contact formation and the initiation ofFtransfer was found. The conjugation process has no effect on the multiplication of the recipient cell.In liquid medium newly infectingF-particles multiply faster than the host cell.The experiment strongly suggests that the number of episomes per cell is small and that the distribution of theF-factor among the daughter cells is non-random.