Three genetically distinct ochre suppressors have been identified in a strain of Escherichia coli B/r, all of which suppress a tyrosine auxotrophy and classify as class 2 by phage suppression pattern. One ochre suppressor, which was obtained by conversion from a class 2 amber suppressor, and a second ochre suppressor obtained directly from the non-suppressing parent, were found to have separate map locations, though a peculiar phenotype with regard to a leucine auxotrophy is exhibited by strains carrying either suppressor. We suggest that both suppressors correspond to separate genes for glutamine-inserting tRNA. A Leu+ mutant of a strain carrying one of these suppressors was studied and was found to contain a further nonsense suppressor having amber-suppressing activity at a reduced level. We suggest that this suppressor might result from a mutation in another part of the translational machinery concerned with glutamine insertion. The third ochre suppressor has no effect on the leucine auxotrophy and mapping data suggest that it may be supL, an ochre suppressor probably inserting a different amino acid, from glutamine.

Mutants of Pseudomonas aeruginosa tolerant to the action of trypsinsensitive aeruginocins can be readily isolated. They are found to be heterogeneous for a range of phenotypic characteristics (including the pattern of membrane protein components in polyacrylamide gel electrophoresis), response to bacteriophages (including both plaque formation and the ability to be lysogenized), sensitivity to various toxic agents, colonial morphology, and cellular morphology. The nature of these changes strongly supports the view that the mutants examined have undergone alteration in membrane structure. A limited genetic analysis indicates that at least two chromosomal regions are involved.

Studies of the growth and composition of Q-strain mice selected over 20 generations for high and low body weight at 6 weeks of age, and their unselected controls, were made on livers and kidneys of males from the five selection replicates A, B, C, D and F. Differences in growth rate between Large and Small QD mice were confirmed from 2 to 9 weeks of age, but were greatest in the third, fourth, sixth and seventh weeks. Total amounts of dry matter, protein, free amino acids, bulk RNA and ribosomes were increased or decreased from control values in proportion to organ weight. A less-perfect relationship between DNA content and organ weight suggested that some small changes in average cell mass had accompanied the main change in cell number in organs from the selected lines. Absorbance profiles of polyribosomes from both organs were identical in selected and control mice: selection had not operated on the proportion of single (currently inactive) ribosomes. Attempts to relate the observed differences in growth rate in QD mice to differences in the rate of protein synthesis produced an unexpected result: incorporation of radioactively labelled amino acids was consistently higher in the organs of the Small mice. Measurements of rates of protein turnover, and calculated rates of protein degradation, suggested that protein might also be degraded more rapidly in the small mice.

The flanking markers of wild-type recombinant spores originating from crosses of spore colour mutants in Sordaria brevicollis were analysed. Recombinant asci were of two main types – either with one or with two wild-type spores. In most crosses the behaviour of flanking markers was significantly different for these two types of recombinant asci. The main differences were in the polarity of gene conversion (as inferred from parental outside marker combinations) and in the frequency of recombinant outside markers. These differences were interpreted in terms of hybrid DNA models of recombination and correction of heteroduplex DNA.

The variation in the response to selection was studied by replication of selected lines. A random-bred strain of mice was divided into six replicates. Two-way selection for 6-week weight was applied in an identical manner to each replicate, and each had an unselected control. Each line (6 large, 6 control, 6 small) was maintained by minimal inbreeding with 8 single-pair matings. The overall mean responses, both up and down, were linear and very regular for ten generations, with realized heritabilities of 40% upwards, 33% downwards and 37% for the divergence. The separate replicates, however, differed greatly in their realized heritabilities, with upward selection ranging from 25 to 46%, and downward selection from 16 to 50%. The theoretical prediction that, because of genetic drift, the standard error of a realized heritability is underestimated by the standard error of the regression of response on cumulated selection differential was borne out in this experiment. The empirical standard error, calculated from the observed variance between replicates, was more than twice as great as that of the regression. The empirical standard errors showed that the asymmetry between upward and downward responses was not significant. The variation between the replicates was ascribed mainly to random drift, which may seriously influence the conclusions about the realized heritability and the asymmetry of response that would be drawn from a single experiment with the population size of one of these replicates. After 23 generations of selection the large lines were approaching limits, and the limit appeared to be at the same level in all. The small lines showed an undiminished realized heritability after 23 generations, but the selection differentials were then so small that little progress was made. There was evidence of counter-acting natural selection. All aspects of productivity – proportion of fertile matings, litter size and weaning rate – declined in the control lines. The overall productivity of the large lines was a little below the controls, and that of the small lines was reduced to about half the level of the controls. The separate replicates differed from each other significantly in all the components of productivity.

A male common shrew trapped at Bonn/BRD was found to have a karyotype differing from the normal karyotype by a dissociation of a large metacentric chromosome. Similar findings are hitherto unknown in mammals.

Serum and urine samples from seven recombinant inbred mouse strains, derived from a cross between BALB/c and C57BL/6, were examined to determine the immunoglobulin heavy chain (IgCH) and the major urinary protein (MUP) allotypes. CXBG and CXBJ exhibited the same IgCH alleles as did BALB/c; the others resembled C57BL/6, thus providing no evidence of crossover types. Comparison of the Mup and brown coat colour (b) alleles (both on linkage group VIII) revealed that three of the strains resemble BALB/c and two resemble C57BL/6, whereas the CXBE and CXBI strains are crossover types.

A variety of Klebsiella strains examined all show either a strong (ML+) or a weak (ML−/+) lactose-positive phenotype on MacConkey agar. ML+ Klebsiellae have about 10 times the β-galactosidase activity of ML−/+ strains in cultures both induced and not induced for this enzyme. Of 14 ML+ strains of diverse origin tested, at least 13 carry a lac operon on a plasmid which can be transferred to Escherichia coli. The seven plasmids so far studied in detail all belong to the F compatibility group but are unable to promote their own transfer. To explain these results it is suggested that the Klebsiella group derive from a common ancestor with a chromosomal lac operon of low efficiency, which was made good by the acquisition of a lac operon from another bacterial strain, probably E. coli: the new lac genes remained as a plasmid, possibly because they could not be integrated in the new host.