Possible Photoregulation by Flavoproteins

Abstract

A wide variety of biological phenomena are blue light-dependent, responding to light of wavelengths below 500 nm. This topic has been the subject of many review articles; a recent one by Schmidt (1) is particularly useful, both in its description of the phenomena and in exploring the basis of the possible molecular mechanisms involved. Many individual examples of blue light photoreception will be dealt with in detail at this meeting; I will simply list here those examples where action spectra permit a reasonable conclusion about the nature of the photoreceptor. There now seems to be general agreement that flavin is the blue light photoreceptor (1). This conclusion is based on several lines of evidence. The action spectra of most blue light dependent biological phenomena show a prominent band around 450 nm, often with a distinct shoulder in the region of 470-480 nm. This spectral characteristic is shared by two classes of widely spread biological compounds, flavins and carotenes. In addition most action spectra also exhibit a near-UV band, in the region of 350 nm. There is however much less consistency about the shape, position and “intensity” of this band, a fact chiefly responsible for the past uncertainty about whether flavin or carotene is the blue light photoreceptor. Flavins do possess such an absorption band, as does cis-ß-carotene. However trans-ß-carotene does not, and the biological occurrence of cis-ß-carotene is apparently quite rare. Particularly important is the failure to detect this form in one of the archetypal organisms exhibiting blue light photoresponse, Phycomyces (2). Another important piece of evidence is that carotene-deficient mutants of various biological species generally exhibit the same blue light responses as the wild types (3-5). Finally while carotene is present in high concentrations in the growing avena coleoptile, it has been shown by microchemical techniques to be absent from the apex, which is the most photosensitive zone of the coleoptile (6).