the MAK and the freon columns. A typical elution profile of the bacterial leucyl-tRNA on the latter column is shown in Fig. 3. There were at least four major peaks in the bacterial system, but only two were detected in the avian system. Moreover, the elution profiles of adult and embryonic leucyl-tRNA’s appeared to differ slightly. The two embryonic peaks were further apart. However, the significance of this difference is not clear at the moment. Thus, the analysis of aminoacyl- tRNA’s from immature erythrocytes of chick embryos and from reticulo- cytes of adult chickens by chromatography on the MAKand freon columns has revealed that the elution pattern for methionyl-tRNA changes during development, whereas the patterns for the four other amino acids studied remain essentially unaltered with a possible difference for leucyl-tRNA. However, it is possible that changes in other untested tRNA’s may exist. The fact that remarkably similar elution patterns of the other aminoacyl-tRNA’s were observed apparently eliminates the possibility of ribonuclease activity as a major factor in the observed change in methionyl-tRNA. In a later experiment, we examined the embryonic methionyl-tRNA with the use of the same 14C-methionine sample as we had used for the adult tRNA in Fig. 2. Because the expected embryonic. pattern was observed, it seemed unlikely that the difference illustrated in Fig. 2 is due to contamination in the 3H-methionine. The identical elution profiles obtained either by 1C-, 3H-, double-, or single-labeling techniques (as in the case of tyrosyl-tRNA) demonstrated that these profiles are re- producible and reliable. The actual mechanism and biological significance of this change are not clear at the moment. The observed modification in methionyl-tRNA dur- ing development may be a change of either the tRNA molecules or of the specificities of the synthetases involved. also be involved in protein synthesis at aminoacyl-tRNA The tRNA may the regulation of the translational level (7). Moreover, our finding is of particular interest because N-formy]l- methionyl-tRNA from E. coli plays a crucial role in the initiation of protein synthesis in bacteria (7). Two methionyltRNA’s of E. coli have been reported (72); one of these can be con- verted to N-formyl-methionyl-tRNA, and the other does not accept formyl 1332 groups. Our finding that methionyltRNA’s are modified during development agrees with the expectation based on the model of regulation that a change in tRNA molecules may lead to an alteration in their functional capacity and thus may affect translation. JOHN C. LEE VERNON M. INGRAM Department of Biology, Massachusetts Institute of Technology, Cambridge 02139 References and Notes 1. N. Sueoka and T. Kano-Suecka, Proc. Nat. Acad, Sci. U.S. 52, 1535 (1964); B. N. Ames and P, Hartman, Cold Spring Harbor Symp, Quant. Biol. 28, 569 (1963); G. S. Stent, Science 144, 816 (1964). 2. T. Kano-Sueoka and N. Sueoka, J. Mol. Biol. 20, 183 (1966). 3. L. C. Waters and G. D. Novelli, Proc. Nat. Acad. Sci. U.S, 57, 979 (1967). 4. A. D. Kelmers, G. D. Novelli, M. P. Stulberg, J. Biol. Chem. 240, 3979 (1965). 5. I, Kaneko and R. H. Doi, Proc, Nat, Acad. Sci. U.S, 55, 564 (1966). 6. J. F. Weiss, A. D. Kelmers, M. P. Stulberg, Fed. Proc. 26, 2667 (1967). 7. L. Felicetti. B. Colombo, C. Baglioni, Biochim. Biophys. Acta 119, 120 (1966). 8. H. Fraenkel-Conrat, B. Singer, A. Tsugita, Virology 14, 541 (1961). 9. Escherichia coli B was obtained from Calbiochem, Los Angeles, California. 0. G, A. Bray, Anal. Biechem. 1, 279 (1960). 1. K, A. Marcker and F. Sanger, J. Moi. Biot. 8, 835 (1964). 12. B. F. C, Clark and K. A. Marcker, ibid. 17, 394 (1966); D. A. Kellogg, B. Doctor, J. Loebel, M. W. Nirenberg, Proc, Nat. Acad. Sci. U.S. 55, 912 (1966), 13, We thank Miss Joanne Wirsig and Mrs. Leslie D. Schroeder for technical assistance. Supported by grant AM 08390 from NIH. 25 October 1967 Bn A Mutagenic Effect of Visible Light Mediated by Endogenous Pigments in Eugiena gracilis Abstract, Mutant cells lackng chlorophyll, chloroplasts, and chloroplast DNA were produced by irradiating Euglena gracilis in aerobic conditions with visible or red light (greater than 610 nanometers) of an intensity equivalent to that of direct sunlight. The photosensitizer is apparently the endogenous chlorophyll present in the chloroplasts. These mutants are comparable to those induced by ultraviolet light, x-rays, heat, or streptomycin. Our findings indicate that visible light can serve as a mutagenic agent in the absence of exogenous photosensitizers, thus directly effecting the course of evolution of organisms containing chlorophyll. .In the presence of a suitable exogenous photosensitizing dye, cells exposed to visible light and air display a photodynamic action, which can be lethal (J) or mutagenic (2) in nature. In the latter case the photosensitizer can be preferentially bound to nucleic acids (3) and may act directly as a “photomutagenic” agent, whereas, in other cases, the mutagenic mechanism is not clearly understood (4). In the absence of an exogenous photosensitizer, it has not been possible to demonstrate mutagenesis with visible light under normal physiological conditions. Kaplan and Kaplan (5) have reported the appearance of S-mutants of Serratia marcescens from cells which had been initially dehydrated and then rehydrated and exposed to visible light. Except for this experiment, it is generally held that light with wavelengths above 300 nm has primarily lethal action but very little mutagenic action (6), although there is no doubt that near-visible light (320 to 400 nm) can be mutagenic (7). The conditions which limit mutagenesis with visible light include the absence of endogenous photosensitizers or the development of suitable protec- tive mechanisms such as those which operate to prevent the damage caused by aerobic photosensitivity (8). We report here the induction of a mutation (that is, a stable, heritable change ex- pressed in the phenotype) by visible light in Euglena gracilis in the absence of exogenous photosensitizers. Euglena gracilis var. bacillaris was cultivated on Hutner’s medium 3.5) (pH with continuous shaking under visible light (275 lumen/m*) (9). Cells taken during the logarithmic phase of growth were transferred to flat-sided tissue-culture flasks and immersed in an aquarium tank maintained at 23°C. Air was bubbled through the flasks for the duration of the experiment, both to insure adequate aeration and to avoid settling of the cells. Illumination was by means of a Sylvania Sun Gun II (650 watts), and the intensity was de- termined with a Yellow Springs In- dustry radiometer, Model 65. For those experiments involving red-light irradiation, a Corning C.S. No. 2-61 glass filter which transmits light only above 610 nm, was placed in front of the culture flask. Illumination was continued for as long as 6 hours, during which time the temperature within the flask, monitored with an electronic thermometer, was SCIENCE, VOL. 158 “a fi