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G. A. JACOBY 1964 2. There is a lag in growth and oxygen uptake on transferring cells from growth on glucose to an amino acid medium during which the enzymes for amino acid catabolism are formed. 3. In the presence of glucose, or any of a variety of alternative energy sources, adaptation for amino 'acid breakdown is repressed. 4. The effect of glucose on the induction of individual enzymes in the catabolic pathways of tyrosine and histidine has been measured with cellfree extracts. 5. p-Hydroxyphenylpyruvate hydroxylase and homogentisate oxygenase, the second and third enzymes of tyrosine degradation, are repressed by glucose, but tyrosine transaminase, the first enzyme, is neither induced by tyrosine nor repressed by glucose. 6. Histidase and urocanase, the first enzymes in the histidine pathway, are repressed by glucose. 7. Utilization of tyrosine or histidine by fully adapted cells is not decreased in the presence of glucose or other metabolic repressors except acetate. 8. Induction and repression of amino acid oxidation are complementary control mechanisms by which the large enzymic potential of this organism is regulated. I am indebted to Dr J. Mandelstam for valuable criticism and advice. This investigation was supported by a U.S. Public Health Service fellowship (no. EPD-18 124) from the National Institute of Allergy and Infectious Diseases. we have found that 8m-urea is not necessary for solution of the protein. The percentage of protein dissolved was determined from the dry weights of the washed residues. Studies on the effect of temperature on the rate of solution gave results almost identical with the first method, i.e. an activation energy of about 13 kcal./mole. The procedure adopted to prepare soluble proteins was as follows: two solutions, containing (i) CuSO4 (01M) plus NH3 (0-5N), Vol. 92 9

It has been shown (Fraser & MacRae, 1963) that solutions of unfractionated feather-keratin derivatives can be dried down into films that give welloriented X-ray-diffraction patterns containing many of the features associated with the microfibrillar structure of the native material. It was found that similar films could be obtained from the fractions described by . In most cases spontaneous birefringence developed around the edge of the film during drying. The X-ray-diffraction patterns yielded by these films when the X-ray beam passed parallel to the surface were similar to those of the unfractionated derivatives at low angles of diffraction, with a prominent meridional reflexion at 23A and equatorial reflexion at 33A. At wider angles the fractionated materials show strong layer lines at 4*8 and 2-4k and a series of equatorials which index as orders of a 33i spacing.The electron micrographs shown in Plate 1 were obtained by spraying a 1:1 (v/v) mixture of 0 05 % protein solution and 1 % phosphotungstate, pH 5 6, on to grids covered with a carbon-collodion fihm and using the negative-staining method of Brenner & Horne (1959). It is apparent from Plate 1 that, on drying, the fractionated derivatives spontaneously polymerize to form fibrils. The apparent thickness of the fibrils varies from 40 to 60 A at their narrowest part (arrow in Plate 1 b) to approx. 130A at their widest part, suggesting that they are helical with a pitch of approx. 1500 k. At first sight the fibrils appear to consist of two filaments coiled around each other to give a two-strand rope, but a ribbon containing three strands cannot be excluded at present. A feature is the tendency of the fibrils to aggregate laterally in precise register.