Max Delbrück

PHOTOTROPISM IN PHYCOMYCES MUTANTS LACKING β-CAROTENE

Abstract. β‐carotene and riboflavin are considered as the major candidates for the photoreceptor for physiological responses to blue light in Phycomyces and a number of other organisms. Mutants of Phycomyces blocked in all six steps of the biosynthesis of β‐carotene from phytoene contain no detectable β‐carotene (less than 4 times 10‐5 of wild‐type amount) but exhibit phototropic responses identical to wild‐type. Moreover, wild‐type Phycomyces, while abundant in trans‐β‐carotene, contains no detectable cis‐β‐carotene, sometimes proposed as a photoreceptor candidate on the basis of the close similarity of the cis‐species absorption spectrum in the near UV region to many action spectra for blue‐light responses. These results indicate that β‐carotene cannot be the photoreceptor for phototropism in Phycomyces.

Light-induced carotene synthesis in Phycomyces

Blue light stimulates the synthesis of β-carotene in the fungus Phycomyces. It appears that the photoregulation of β-carotene synthesis in Phycomyces is under (at least) dual control. The dually controlled nature of light-induced β-carotene synthesis in Phycomyces is supported by: (1) the possible biphasic appearance of fluence-response data which suggest the superposition of two components, one responsive to low light fluence (saturation at ∼20 to 40 J/m2) and one responsive to high light fluence (saturating at ∼1000 J/m2); (2) the differential sensitivities of the low- and high-fluence components to the transcription and translation inhibitors, actinomycin D and cycloheximide, respectively; (3) the existence of mutants defective in the high-fluence component but normal in the low-fluence component.

LIGHT-INDUCED CAROTENE SYNTHESIS IN MUTANTS OF PHYCOMYCES WITH ABNORMAL PHOTOTROPISM

Mutants of Phycomyces. abnormal in their phototropic responses (the mad mutants), have been tested for their responses in light‐induced carotene synthesis (LICS). The amount of carotene synthesized at any given fluence is significantly lower in the madA, madB and madD mutants than in the wild type. The amount of carotene is not lower in other mad mutants (madC, madE, madF and madG). The double mutant mad A madB and the triple mutant mad A madB madC show stronger effects. The wild type strain, as well as those carrying a single mad mutation (madA and madB) or those carrying two or three mad mutations (madA madB. madA madB madC) show closely similar sensitivity to LICS. This contrasts with phototropism and photoinitiation of sporangiophores which are sensitive to extremely small signals in the wild type and in which the madA mutation decreases the sensitivity by nearly a factor of 104 and madB mutation by a factor of 105. It appears that LICS does not share the signal amplification mechanisms characteristic of the other two responses.

Carotenogenesis in Phycomyces

Abstract Time course studies of carotenoid production and of mycelial growth in liquid cultures of Phycomyces blakesleeanus wild type [NRRL 1555 (−)], red mutants C9, C10 and C13 and the heterokaryon C2 * C9 are reported. The ratios of the concentrations of lycopene, γ-carotene and β-carotene in the red mutant C13 and in the heterokaryon C2 * C9 during the growth periods were measured. In these strains the concentration of lycopene is close to its final value after 2 days of growth, at a time at which β-carotene is just beginning to be produced. It is suggested that the β-carotene produced late is possibly synthesized via β-zeacarotene.