1. Individual genes of a multigenic system of resistance to actidione in yeast were analysed in a series of crosses involving resistant strains and sensitive strains.

2. Resistance conferred varied in the range 0·5 p.p.m. to 20 p.p.m.

3. Positive interaction was seen between resistance genes in recombinant strains.

4. Recessive modifier genes were found which do not themselves confer resistance but which modify the activity of specific resistance genes in a positive way, that is towards a greater degree of resistance. Both a two-fold and a five-fold increase in resistance is seen depending on the gene combination.

5. In selection experiments, strains resistant to relatively high concentrations of actidione (1000 p.p.m.) were obtained but only by selection on media containing successively higher concentrations of the drug, that is, by the step wise build up of multiply mutant forms. Mutations of interacting resistance genes and specific modifier genes were obtained by selection methods including pretreatment of cells with ultra-violet light.

Under suitable experimental conditions, short denatured DNA fragments may be bound by high molecular weight denatured DNA immobilized in agar (‘long DNA’). The proportion of fragments bound depends in part upon the degree of taxonomic relationship between the organisms from which the two DNA components are taken. Before the method can be used as a measure of genetic relationship between closely related species or between intra-specific groups, its resolving power requires to be increased. The present study explores some modifications of the method from this point of view, using the degree of binding between DNA from laboratory mice and rats as a model.

Long rat DNA was less effective than long mouse DNA in binding short DNA fragments from either source. It was therefore necessary to test reciprocally, using both short mouse and short rat DNA on long DNA from each species. Combining reciprocals gives a measure of the proportion of fragments bound in within-species and in between-species combinations. With rats and mice, between-species binding was at about 70% of the within-species level. The discrepancy could be increased, and hence the discriminating power of the method improved, if the binding reaction was partially or totally ‘blocked’ with an excess of unlabelled DNA fragments in an appropriate species combination.

Even when short and long DNA came from the same source, only about a third of the fragments were bound. The bound and unbound fractions were recovered, and each reincubated with a fresh sample of long DNA. The two fractions then behaved very differently, suggesting that the original DNA preparation is heterogeneous. The initially unbound DNA showed a very low binding ability in subsequent incubations and may be considered virtually ‘unbindable’.

When the first incubation is a between-species one, the unbound fraction should include not only this ‘unbindable’ DNA, but also any DNA characteristic of that species alone, and not held in common by both species. If it could be isolated, this DNA fraction might be expected to discriminate maximally between the two species. By using three successive cycles of incubation, a fraction was obtained which showed considerably improved discriminating power, in that it showed only about 30% as much binding in between-species as in within-species combinations.

Six experimental populations of Drosophila pseudoobscura were maintained in three different environments for almost four and a half years. All the populations have originally descended from the same founders. The populations were examined for evidences of genetic divergence. The longevity of the flies from four of these populations, and their F1 and F2 hybrids, was studied at 16°C. and at 27°C. The results indicate that the gene pools of some of the populations have diverged from each other.

1. The maintenance of mu particles and metagons from Paramecium aurelia (stock 540, syngen 1) in another ciliate protozoan Didinium nasutum has been shown to occur.

2. It has been shown that mu particles could not be supported in Didinium without metagons. One particular strain of Didinium has, however, never been able to support mu even when metagons were present.

3. The continued production of metagons in Didinium was shown to take place even when the Didinium was fed on killer Paramecium of genotype m1m1m2m2.

4. After destruction of the metagons in Didinium by ribonuclease there was no subsequent reappearance of metagon activity.

5. Metagons and mu particles were eliminated from Didinium during encystment.

6. It is considered that the increase of metagons in Didinium is due to self-replication. In Paramecium the metagons replicate only slowly or not at all.

7. It is concluded that the metagon is a gene product in Paramecium which takes on at least one other property, replication, when introduced into Didinium.

Linkage data, obtained by a combination of selective analysis of haploid recombinants and analysis of segregating heterozygotes, are given for twenty-eight loci in Streptomyces coelicolor A3(2). This brings the total known loci for the organism to thirty-nine. The two linkage groups previously recognized remain separate, and their lengths have been increased to about 60 and 70 recombination units respectively. Whether the two linkage groups correspond to two chromosomes remains an open question.

Three further examples of close linkage of pairs of functionally related loci have been found (a trio of such loci was already known), and three other pairs provide possible examples of the same phenomenon. Some loci which are clustered in Salmonella are unlinked in Streptomyces.

1. Heterozygous (K/k) killer paramecia containing kappa particles from stock 51, syngen 4 of Paramecium aurelia were passed through autogamy, and the kk offspring tested for killing ability and for presence of stainable kappa particles at various fission stages.

2. After eight fissions, a small proportion of the paramecia were non-killers, and a few lacked stainable particles. After the 12th fission nearly all cells lacked kappa.

3. Mild starvation of the kk kappa-bearing paramecia resulted in complete loss of kappa and conversion to non-killers, even when applied to animals only one fission after the loss of gene K at autogamy.

4. The results obtained here show some discrepancy with data obtained earlier, both with kappa, and with mu particles in stock 540, syngen 1. The reasons for this discrepancy are at present not clear.

In zygotes from matings between Hfr, F′ or F+ donors of E. coli K and F− recipients of E. coli B the restriction of λ. K and λ.C is 100 times less than in unmated B cells. This decrease in the capacity of zygotes to restrict phage λ is transient and is not related to the transfer of genes controlling host-induced modification during the cross.

Congenital defects due to genes are the static end results of abnormal processes in embryonic development. Are these ‘burnt out’ and no longer in operation later in life, or do they continue as active physiological processes? To answer this question, thirteen genes with comparatively trivial morphological effects have been subjected to a screen test. For eight of them (bp, f, Ts, Sd, un, se, Os and Ph) a prima facie case has been established for the existence of continuing processes of one kind or another. For the remaining five genes (dn, Pt, T, vt and Miwh) there is no such evidence though the possibility remains that such processes might be detectable by different means.

A total of eighty-seven revertants were induced by ultra-violet light in an am3 strain. All of these revertants appear to be the result of mutation at sites in or close to the am locus. Fourteen of the eighty-seven revertants were partial revertants in that under some conditions of assay they possessed low glutamate dehydrogenase activity compared with the wild-type although their growth rate was similar to that of the wild-type. Enzyme extracts of thirteen of the partial revertants were assayed for glutamate dehydrogenase in various ways in order to establish qualitative distinctions between different kinds of mutant enzyme. On the basis of these tests six different groups were established, of which one contained six revertants, one three and the others one. All except one of the mutant enzyme types showed a marked activation when incubated with α-oxoglutarate plus NADPH2, and all of these had Michaelis constants for ammonium ion much higher than is found for the wild-type enzyme. The remaining group of three revertants gave, at first, no enzyme activity in any of the assay systems. Two of these (the third was not tested) were shown to produce an enzyme variety which becomes quite inactive in phosphate buffer at pH 8·0 but can be fully activated by the addition of ethylenediamine tetra-acetic acid. Forced heterocaryons between each of six partial revertants and eleven am mutants were made and the resultant sixty-six heterocaryons assayed for glutamate dehydrogenase activity. The partial revertants differed among themselves in their complementation characteristics. Some complemented with none of the am mutants, some with am1 only, and some with am1 or with am7. The complementation tests confirmed the differences established by the enzyme studies. The data presented here, together with previous work, demonstrate that ultra-violet light induced mutation in an am strain can result in at least eight types of revertant differing from each other in respect of the glutamate dehydrogenase variety which each can produce.

Phage F116 transduces sufficiently large fragments of the Pseudomonas aeruginosa chromosome for co-transduction of linked markers to occur. The linkage relationships of 176 auxotrophic mutants have been examined by means of this technique, following characterization of the mutants for their nutritional requirements and by other biochemical means. Thirty-two different loci from fourteen biosynthetic pathways have been differentiated by such combined means. Unlike other bacteria, the clustering of related loci of a biosynthetic pathway appears to be rare. This situation was confirmed for the leucine, methionine, adenine, isoleucine plus valine and arginine pathways. No evidence for linkage of markers within the histidine, proline, cysteine and uracil pathways has been found but further data is needed to completely confirm this conclusion for these groups of markers. The importance of these findings in relation to the genetic control of enzyme biosynthesis in P. aeruginosa is discussed. For markers of the tryptophan pathway, two are possibly linked but the other three are definitely not linked. The only confirmed linkage of related markers was between a homoserine locus and a threonine locus.

Ten U.V.-induced mutants at the adenine-1 locus of Schizosaccharomyces pombe differ not only in their intragenic location but also in their spontaneous, HNO2- and U.V.-induced frequencies of reversion to adenine-independence. Seven mutants reverted with HNO2 and of these only two reverted also with U.V.; the other three did not revert with either HNO2 or U.V.

Genetic analysis of revertants showed that they could be due to suppressor mutations at a locus or loci far from the adenine-1 locus, or to events extremely close to, if not identical with, the original site of mutation. Many distant suppressors were found among spontaneous revertants of some adenine-1 and adenine-7 allles, but such suppressors appear to be less frequent among the five adenine-8 mutants tested. There are large differences between different sites at the same locus in the proportion of their spontaneous revertants due to distant suppressor mutations. Nitrous acid-induced revertants of adenine1-, adenine-7, and adenine-8 mutants were almost all of a type containing no detectable suppressor mutation. In the three cases studied, U.V.-induced revertants could be either predominantly of a suppressor or non-suppressor type, depending on the particular mutant examined.

Striking examples of mutagen specificity were found in which a particular mutant gave mostly suppressor-type revertants with one mutagen, while with another mutagen revertants were mainly of a type containing no detectable suppressor mutation.

A number of mutants of E. coli K 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.

Jacob & Wollman (1961) produced interruption of chromosome transfer during conjugation in E. coli K-12 by exposing conjugating pairs to high shear forces created by a propeller-type mixer (a Waring blendor). In our laboratory, efforts to use this method were not completely successful because of the relatively long times required to ‘blend’ each sample and the large variations from sample to sample in the efficiency of separating mating couples (‘blending efficiency’). The variations in blending efficiency were thought to be due to lack of homogeneity of mixing in the blendor chamber. We have obtained much more satisfactory results by using a vibratory mixer which shakes the entire sample chamber and thoroughly agitates all the cells. Furthermore this vibratory blendor is much easier to use and permits taking at least twice as many samples per unit time, as compared to the usual blendor method.

A multi-step Chloramphenicol (CM)-resistant derivative of an RC-stringent strain of Escherichia coli auxotrophic for threonine and leucine was resistant also to Aureomycin (AM) and Puromycin (PM). All three antibiotics released the repression of RNA synthesis due to amino acid starvation in the CM-sensitive parent strain, their relative activities being about 1:10:100 for AM: CM: PM. High doses of AM and CM failed to induce RNA synthesis. The CM-resistant strain required greater concentrations of each antibiotic than the sensitive strain to induce the same level of RNA synthesis, and appeared to be about one hundred times, ten times and five times more resistant to CM, AM and PM, respectively, than the sensitive strain.

The effects of P32 disintegration in the Hfr donor cell on the genetic recombination of Escherichia coli K 12 have been studied. The dose-response relationship for the transfer of markers at a distance x from the chromosomal origin, N/N0 = e−αxD, which summarizes the findings of Fuerst et al. (1956) for early markers of Hfr H, has been found to hold true to a good approximation for more distal selected markers in this strain, as well as for selected markers of Hfr CS, which transfers its chromosome in the opposite direction. The inheritance by recombinants of non-selected male markers located between the origin and the selected marker is not affected at all by prior P32 dose to the Hfr. The sensitivity of each Hfr strain studied to the lethal effects of P32 decay corresponds to the sensitivity to transfer effects of a male marker located about one-third of the total chromosomal length away from the origin. Genetic and lethal effects in Hfr H showed the same dependence on storage temperature during P32 decay. Our data, therefore, supports the concept of a common origin for the lethal and genetic effects of P32 decay.

The quantitative predictions of current models for chromosome transfer with regard to radiation mating experiments have been examined. A simple model, involving no DNA synthesis, as well as the replicon model of Jacob et al. (1963) predict exponential marker inactivation curves of the kind found experimentally. The model of Bouck & Adelberg (1963) predicts a more complex dose dependence, but one not nearly so different from that of the other two models as was anticipated by Jacob et al. It appears unjustified to attempt to exclude any of the three models on the basis of presently available data from radiation mating experiments.

By performing matings using various Hfr strains having different origins of transfer, it has been observed that male chromosomal markers located very near the origin of transfer in conjugation in E. coli appear in recombinants with a lower frequency than expected from an extrapolation of the transfer gradient. The effect is greatest for markers closest to the origin, and becomes negligible for markers transferred more than 4–5 min. after the origin. The results suggest that in the zygote a random cross-over between male and female genomes is necessary somewhere between the origin and any male marker in order for that marker to appear in a viable recombinant. It was also deduced that the entry time for the origin itself occurs after a 3–5 min. delay after mixing male and female cells together.

Chloramphenicol (CM), Aureomycin (AM) and Puromycin (PM) induce RNA synthesis in RC-stringent Escherichia coli starved of a required amino acid. This fact has been used to develop a method for comparing the levels of resistance of single-step CM-r mutants to the three antibiotics. Three levels of resistance to each antibiotic were found among four mutants selected in a single CM-s strain. The mutant with the highest CM resistance has the lowest AM resistance, and vice versa, while the level of PM resistance was not correlated with that of either CM or AM. The four mutants all differed from each other in their patterns of resistance to the three antibiotics.