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MAX LUDWIG HENNING DELBRÃCK 93 mystery. The really important achievement of the Group during this romantic phase of the growth of molecular biology was "the introduction into microbial genetics of previously unknown standards of experimental design, deductive logic, and data evaluation. These procedures had led to final and definitive settlement of matters that had been under dispute for ten or more years" (17). THE PHYCOMYCES PERIOD (1953-81) Max was basically a theoretician who lived to search for neat models and hypotheses to explain complex phenomena. About 1950, after discovery of the phage eclipse phase but before the Hershey-Chase experiment, he became interested in sensory perception and its transduction into physiological activityâ a phenomenon more relevant to the complex behaviour of higher creatures. He also thought that, by then, phage research was "in good hands." His first choice of a simple model organism was the purple bacterium, Rhodospirillum, which is not only photosynthetic but also phototactic, swimming towards a light source. Max was co-author of a general article on Rhodospirillum (1951b) in which the responses of this organism to light were compared with those of nerve fibers to electrical stimuli. However, after some early experiments he forsook this organism in favour of a simple fungus, Phycomyces. Phycomyces has a non-septate mycelium which sprouts large aerial stalks called sporangiophores, each crowned by a spherical sporangium containing many thousand spores. The attractiveness of this organism as a model for studying perception and response lay in the reactions of the rapidly growing sporangiophores to many stimuli. For example they grow towards the light (phototropism), against gravity (geotropism), into the wind (anemotropism), and away from nearby objects (avoidance response). On the other hand,
MAX LUDWIG HENNING DELBRÃCK 94 Phycomyces does not naturally form heterokaryons and produces multinucleate asexual spores, while the sexual cycle, involving two mating types that initially were far from isogenic, takes several months to yield recombinant progeny. Thus the organism lacks the ease and refinement of genetic analysis that made some other microbial systems, such as Escherichia coli, ideal tools in molecular biology. Early studies of phototropism were initiated at Cold Spring Harbor in 1953 and the next year Max persuaded Werner Reichardt, then studying insect optomotor responses at TÃ¼bingen, to join him in his Phycomyces project. This partnership resulted in a classic paper (1956b) proposing a kinetic model of adaptation to light that proved influential for other sensory systems, although it has recently been shown to be adequate only for dark adaptation in the normal intensity range in the case of Phycomyces (E. Lipson, pers. comm.). Thereafter a Phycomyces Group grew slowly, recruitment being mainly from physicists with no defectors from the Phage Group apart from Max himself. The Cold Spring Harbor workshops, each lasting about two months and beginning in 1964, attracted many participants from abroad who spread the gospel. Some regularly visited Cold Spring Harbor or Caltech for periods of collaborative discussions or research, especially from France, Germany, Japan, and Spain. In 1969 the Phycomyces cause was further publicized by a comprehensive review of the whole field, to which 12 members of the group made specialized contributions. In his introduction Max stated, "This review, then, is addressed to those who aim to push sensory physiology to the limits of molecular biology. We believe that what can be learned from Phycomyces is relevant to this next phase of our quest for a mechanistic understanding of life." While agreeing that Phycomyces does not permit analysis of electrical sig
MAX LUDWIG HENNING DELBRÃCK 95 nals, "which sensory physiologists have come to consider the sine qua non of their trade," nevertheless he believed that there is "much room for similarities in earlier stages of the transducer chain . . . and the receptor potentials of animal sensory cells, and it is to these as yet obscure stages that we think Phycomyces work can make a contribution of general relevance" (1969). The adaptation range of Phycomyces to light is about ten orders of magnitude, equivalent to that of the human eye, and sensitivity is specific for blue light (1960). Max's main interest in recent years was the nature of the photoreceptor, the most likely candidate being Ã carotene or a flavin. With Katzir and Presti (1976a) he greatly extended the action spectrum and found absorption in the region of 600 nm which they interpreted as evidence for a flavin chromophore. Subsequently Ã carotene was excluded by the use of mutants in which its synthesis was undetectable (1977a, 1978b). Finally, in his last published paper, Max and his colleagues (1981) found that the substitution of an analogue of riboflavin (roseoflavin, with a distinctive absorption spectrum) in a riboflavin auxotroph produces an equivalent shift in the action spectrum. It thus seems likely that the sporangiophore blue light receptor of Phycomyces is a flavin and not a carotene, although the precise nature of the compound remains unknown (see 15). In the years that have elapsed since the 1969 review, much interesting work and some important technical advances have been made, especially in the field of behavioural genetics. For example, the introduction of a microsurgical technique for making heterokaryons and the development of isogenic mating types have revolutionized genetic analysis. A large number of behavioural mutants have now been isolated, involving photoresponses to sporangiophore development and carotene synthesis as well as various tro