A positively gravitropic mutant mirrors the wild-type protonemal response in the moss Ceratodon purpureus

Wild-type Ceratodon purpureus (Hedw.) Brid. protonemata grow up in the dark by negative gravitropism. When upright wild-type protonemata are reoriented 90°, they temporarily grow down soon after reorientation (“initial reversal”) and also prior to cytokinesis (“mitotic reversal”). A positively gravitropic mutant designated wrong-way response (wwr-1) has been isolated by screening ultraviolet light-mutagenized Ceratodon protonemata. Protonemata of wwr-1 reoriented from the vertical to the horizontal grow down with kinetics comparable to those of the wild-type. Protonemata of wwr-1 also show initial and mitotic reversals where they temporarily grow up. Thus, the direction of gravitropism, initial reversal, and mitotic reversal are coordinated though each are opposite in wwr-1 compared to the wild-type. Normal plastid zonation is still maintained in dark-grown wwr-1 apical cells, but the plastids are more numerous and plastid sedimentation is more pronounced. In addition, wwr-1 apical cells are wider and the tips greener than in the wild-type. These data suggest that a functional WWR gene product is not necessary for the establishment of some gravitropic polarity, for gravitropism, or for the coordination of the reversals. Thus, the WWR protein may normally transduce information about cell orientation. Received: 4 November 1996 / Accepted: 26 November 1996     

Aphototropic mutants of the moss Ceratodon purpureus with spectrally normal and with spectrally dysfunctional phytochrome

Following UV mutagenesis of protonemal tissue of the moss Ceratodon purpureus we have isolated different aphototropic mutant lines that can be divided into two distinct classes. One class, represented by the line ptr1, shows characteristic features of phytochrome chromophore deficiency. ptrl shows negligible photoreversibility (<5% of wild type), whereas immunoblots show normal apoprotein levels. The aphototropic phenotype could be partially restored with biliverdin, a precursor of the phytochrome chromophore. It was found that, whereas in wild type formation of Pfr leads to suppression of gravitropism, there is no such suppression ptrl. In addition, ptr1 shows lower chlorophyll levels than the wild type. These findings indicate that, as expected for a chromophore-deficient mutant, multiple phytochrome effects are lost. The other class of mutants, represented by the line ptr103, shows more specific effects. In this mutant, only phototropism is affected. Suppression of gravitropism, the content of chlorophyll and photoreversibility of phytochrome were similar to those of the wild type.     

Curvature Induced by Amyloplast Magnetophoresis in Protonemata of the Moss Ceratodon purpureus

After gravistimulation of Ceratodon purpureus (Hedw.) Brid. protonemata in the dark, amyloplast sedimentation was followed by upward curvature in the wild-type (WT) and downward curvature in the wwr mutant (wrong way response). We used ponderomotive forces induced by high-gradient magnetic fields (HGMF) to simulate the effect of gravity and displace the presumptive statoliths. The field was applied by placing protonemata either between two permanent magnets at the edge of the gap, close to the edge of a magnetized ferromagnetic wedge, or close to a small (<1 mm) permanent magnet. Continuous application of an HGMF in all three configurations resulted in plastid displacement and induced curvature in tip cells of WT and wwr protonemata. WT cells curved toward the HGMF, and wwr cells curved away from the HGMF, comparable to gravitropism. Plastids isolated from protonemal cultures had densities ranging from 1.24 to 1.38 g cm⁻³. Plastid density was similar for both genotypes, but the mutant contained larger plastids than the WT. The size difference might explain the stronger response of the wwr protonemata to the HGMF. Our data support the plastid-based theory of gravitropic sensing and suggest that HGMF-induced ponderomotive forces can substitute for gravity.The force exerted by gravity is proportional to an object''s volume and density. Therefore, objects denser than the surrounding medium fall or sediment. Much evidence suggests that gravity sensing in higher plants depends on the sedimentation of dense, starch-filled amyloplasts inside specialized cells, so-called statocytes (Sack, 1991, 1997; Kuznetsov and Hasenstein, 1996, 1997b; Balu$\breve{s}$ka and Hasenstein, 1997).Dark-grown protonemata of the moss Ceratodon purpureus are tip-growing cells that are negatively gravitropic, i.e. they grow upward (Fig. ​(Fig.1).1). The wwr mutant (wrong way response) of C. purpureus is positively gravitropic, with reaction kinetics similar to the WT (Wagner et al., 1997). In horizontal WT (Fig. ​(Fig.1)1) and wwr protonemata, amyloplasts sediment in a specific zone located behind the apical dome. Plastid sedimentation is probably responsible for gravity sensing in both genotypes because it precedes curvature and because the recovery of gravitropism after basipetal centrifugation correlates with the return and sedimentation of amyloplasts (Walker and Sack, 1990, 1991; Wagner et al., 1997; Sack et al., 1998). Figure 1Gravitropic curvature and amyloplast sedimentation (arrowheads) in WT protonemata of C. purpureus that were rotated from the vertical to the horizontal 4 to 5 h before fixation.To study further the possible role of amyloplast sedimentation in gravity sensing, it is helpful to displace amyloplasts without reorienting the cell in the gravitational field. This can be achieved by exposing cells to an HGMF, thereby inducing the intracellular magnetophoretic displacement of starch-containing plastids (Kuznetsov and Hasenstein, 1996, 1997a, 1997b).Dense plastids such as amyloplasts and the cytoplasm differ in their chemical composition and physical properties, including their magnetic characteristics. When subjected to a nonuniform magnetic field, magnetically heterogeneous systems experience ponderomotive forces that depend on their relative magnetic susceptibilities (Kuznetsov and Hasenstein, 1996). Therefore, a magnetic field of sufficient intensity and gradient should be able to displace plastids inside the cell and provide an excellent test for plastid-based gravity sensing.If gravity sensing is plastid dependent, negatively gravitropic WT protonemata should curve toward stronger field intensities. In contrast, wwr cells should curve toward lower field intensities or in a positive gravitropic sense, similar to previous experiments with positively gravitropic roots (Audus, 1960; Kuznetsov and Kuznetsov, 1989; Kuznetsov and Hasenstein, 1996) and negatively gravitropic shoots (Schwarzacher and Audus, 1973; Kuznetsov and Hasenstein, 1997b). These experiments suggest that intracellular magnetophoresis is equivalent to plastid-based gravity sensing. However, these experiments were performed with higher plant organs, where the sites for perception and response are different, rather than with single cells that are capable of both sensing and responding to gravity. Moreover, the small size of moss protonemata and the availability of genotypes with opposite gravitropic responses warrants the use of HGMFs to study the possible involvement of plastid-based sensing in C. purpureus. If gravitropic sensing depends on the amyloplast sedimentation, then exposure to a magnetic field should induce both amyloplast displacement and the curvature of the tip cells in directions that are genotype dependent.This hypothesis was tested using several configurations to produce magnetic fields of different intensities and geometries. We report here that exposure to HGMF caused magnetophoretic displacement of amyloplasts and induced curvature in both WT and wwr protonemata in the predicted directions.     

Gravitropism and gravimorphism during regeneration from protoplasts of the moss Ceratodon purpureus (Hedw.) Brid.

Wild-type (WT) protonemata of the moss Ceratodon purpureus grow upwards in darkness (negative gravitropism), whereas protonemata of the mutant, wrong-way response (wwr-1) grow down. Since Ceratodon protoplasts regenerate to form new protonemata, we analyzed whether the direction of filament emergence was influenced by gravity (gravimorphism) and determined the cytological events that correlated with the onset of gravitropism in WT and wwr-1 filaments formed de novo. In the WT the direction of filament emergence appeared to be gravimorphic as more than 66% of the new filaments emerged above the horizontal. In contrast, the direction of filament emergence was random in wwr-1. Tip-growing cells of both genotypes became gravitropic within a total of one to two cell divisions. Gravitropic curvature in wwr-1 was opposite in direction to that of WT, and the timing of curvature was comparable, indicating that the wwr-1 mutation acts during the onset of gravitropic competence. In time-lapse studies of both genotypes, neither a plastid-free zone nor obvious and extensive plastid sedimentation characteristic of mature dark-grown protonemata was observed in the new filaments prior to gravitropic curvature. Thus, it appears that these latter two features are not required for gravitropism in new protonemal filaments from protoplasts. Received: 24 October 1997 / Accepted: 18 November 1997     

Amyloplasts as possible statoliths in gravitropic protonemata of the moss Ceratodon purpureus

The kinetics of gravitropism and of amyloplast sedimentation were studied in dark-grown protonemata of the moss Ceratodon purpureus (Hedw.) Brid. The protonemata grew straight up at a rate of 20–25 m·h^– in nutrient-supplemented agar. After they were oriented to the horizontal, upward curvature was first detected after 1–1.5 h and reached 84° by 24 h. The tip cells exhibited an amyloplast zonation, with a tip cluster of nonsedimenting amyloplasts, an amyloplast-free zone, and a zone with pronounced amyloplast sedimentation. This latter zone appears specialized more for lateral than for axial sedimentation since amyloplasts sediment to the lower wall in horizontal protonemata but do not fall to the basal wall in vertical protonemata. Amyloplast sedimentation started within 15 min of gravistimulation; this is within the 12–17-min presentation time. The data support the hypothesis that some amyloplasts function as statoliths in these cells.This work was supported by the National Aeronautics and Space Administration grant NAGW-780. We thank Professor E. Hartmann and J. Schwuchow for providing Ceratodon cultures, Dr. John Z. Kiss and Jeff Young for valuable discussions, and Professor Rainer Hertel (University of Freiburg, FRG) for bringing this material to our attention.     

The Lazy Mutation in Rice Affects a Step between Statoliths and Gravity-Induced Lateral Auxin Transport

Abstract: Compared to wild type, the lazy mutant in Oryza sati-va L. shows a reduced gravitropic response. In order to locate the lesion in the stimulus-response chain, coleoptile segments of lazy rice were investigated with respect to auxin transport. Gravity-induced lateral movement of radiolabelled indoleacetic acid (IAA) was strongly inhibited by the lazy mutation compared to wild type while uptake and longitudinal transport of IAA, as well as amyloplast sedimentation, were not significantly affected. These findings suggest that LAZY controls a step in the signalling chain between statoliths and auxin secretion.     

Irradiance-dependent regulation of gravitropism by red light in protonemata of the moss Ceratodon purpureus

Apical cells of protonemata of the moss Ceratodon purpureus (Hedw.) Brid. are negatively gravitropic in the dark and positively phototropic in red light. Various fluence rates of unilateral red light were tested to determine whether both tropisms operate simultaneously. At irradiances ≥140 nmol m⁻² s⁻¹ no gravitropism could be detected and phototropism predominated, despite the presence of amyloplast sedimentation. Gravitropism occurred at irradiances lower than 140 nmol m⁻² s⁻¹ with most cells oriented above the horizontal but not upright. At these low fluence rates, phototropism was indistinct at 1 g but apparent in microgravity, indicating that gravitropism and phototropism compete at 1 g. The frequency of protonemata that were negatively phototropic varied with the fluence rate and the duration of illumination, as well as with the position of the apical cell before illumination. These data show that the fluence rate of red light regulates whether gravitropism is allowed or completely repressed, and that it influences the polarity of phototropism and the extent to which apical cells are aligned in the light path. Received: 19 January 1999 / Accepted: 19 March 1999     

Blue light- and genetically-reversed gravitropic response in protonemata of the moss Ceratodon purpureus

In darkness, protonemal filaments of Ceratodon purpureus (Brid.) grow negatively gravitropically (upwards). Red light induces a positive phototropic response mediated by the photoreceptor phytochrome. A red light treatment also has an inhibitory effect on the gravitropic response, an effect also mediated by phytochrome. In this study the effects of blue light on phototropism and on gravitropism were analysed. Unilateral blue light resulted in only a weak phototropic response, but markedly randomised growth direction. Blue light given together with a gravitropic stimulus reversed the gravitropism, changing it from negative to positive (filaments grow downward). The effect of blue light was also analysed with the mutant ptr116, which is defective in the biosynthesis of the phytochrome chromophore, and in a newly isolated mutant wwr2, which is positively gravitropic in darkness. Blue light induced the same reversal of gravitropism in ptr116 as in the wild type, indicating that phytochrome is not involved in this process. In wwr2 the direction of gravitropism was unaltered by the blue light treatment. Light also affects chlorophyll content and the size of plastids, potential statoliths for gravitropism. Red light induced an increase in plastid size and chlorophyll content in the wild type but not in ptr116. Blue light induced a similar change in wild type plastids. It seems as though light-induced alterations of gravitropism are not simply mediated by alterations in plastid properties, and that red light and blue light evoke fundamentally different responses. Received: 11 July 1997 / Accepted: 30 January 1998     

Genetic correlations do not constrain the evolution of sexual dimorphism in the moss Ceratodon purpureus

The trajectory of phenotypic evolution is constrained in the short term by genetic correlations among traits. However, the extent to which genetic correlations impose a lasting constraint is generally unknown. Here, I examine the genetic architecture of life-history variation in male and female gametophytes from two populations of the moss Ceratodon purpureus, focusing on genetic correlations within and between the sexes. A significant negative correlation between allocation to vegetative and reproductive tissue was evident in males of both populations, but not females. All traits showed between-sex correlations of significantly less than one, indicating additive genetic variance for sexual dimorphism. The degree of dimorphism for traits was significantly negatively associated with the strength of the between-sex correlation. The structure of genetic correlations among life-history traits was more divergent between the two populations in females than in males. Collectively, these results suggest that genetic correlations do not impose a lasting constraint on the evolution of life-history variation in the species.     

Life history variation in gametophyte populations of the moss Ceratodon purpureus (Ditrichaceae)

The life cycles of mosses and other bryophytes are unique among land plants in that the haploid gametophyte stage is free-living and the diploid sporophyte stage is ephemeral and completes its development attached to the maternal gametophyte. Despite predictions that populations of haploids might contain low levels of genetic variation, moss populations are characterized by substantial variation at isozyme loci. The extent to which this is indicative of ecologically important life history variation is, however, largely unknown. Gametophyte plants from two populations of the moss Ceratodon purpureus were grown from single-spore isolates in order to assess variation in growth rates, biomass accumulation, and reproductive output. The data were analyzed using a nested analysis of variance, with haploid sib families (gametophytes derived from the same sporophyte) nested within populations. High levels of life history variation were observed within both populations, and the populations differed significantly in both growth and reproductive characteristics. Overall gametophytic sex ratios did not depart significantly from 1:1 within either population, but there was significant variation among families in both populations for progeny sex ratio. Some families produced predominantly male gametophytes, while others yielded predominantly females. Because C. purpureus has a chromosomal mechanism of sex determination, these observations suggest differential (but unpredictable) germination of male and female spores. Life history observations showed that male and female gametophytes are dimorphic in size, maturation rates, and reproductive output.     

Effects of hardening and freezing stress on membrane lipids and CO2 fixation of Ceratodon purpureus protonemata

Membrane lipids and steady-state CO₂ fixation rates were studied in moss protonemata in order to evaluate separately the effects of growth temperature, freezing stress and the achievement of frost hardiness. Protonemata of Ceratodon purpureus (Hedw.) Brid, were grown at 20 and 4°C and parts of both materials were then hardened. The low growth temperature increased the content and unsaturation level of membrane lipids significantly. This did not, however, cause a noticeable increase in the frost hardiness. Nor was the achievement of frost hardiness in this material accompanied by further changes in the amount or unsaturtion level of any membrane lipid class. Cytoplasmic membranes were abundant in both unhardened and hardened materials grown at 4°C, which agreed with the high phospholipid content of these protonemata. The only significant difference in membrane lipids between unhardened and hardened materials was a 50% lower level of trans 16:1 fatty acid in the phosphatidylglycerol fraction of hardened protonemata.
In hardened protonemata monogalactosyldiacylglycerol (MGDG) was the membrane lipid most liable to decrease during the freeze-thaw cycle. The loss of MGDG was accompanied by partial inhibition of CO₂ fixation. Provided the content of phospholipids remained unchanged (freeze-thaw cycle with – 10°C in hardened protonemata), this inhibition was mostly reversible. If loss of the phospholipids also had occurred during the freeze-thaw cycle, as was the case in unhardened material at or below -10°C, CO₂ fixation was severely and nearly irreversibly inhibited after thawing.     

Selective sweeps and intercontinental migration in the cosmopolitan moss Ceratodon purpureus (Hedw.) Brid

The moss Ceratodon purpureus has long been used as a model system in plant development and physiology. However, the molecular population genetics of the species remains virtually unexplored. In this study, we used population genetic analyses of DNA sequence data from three unlinked loci (atpB-rbcL spacer, adk, and phy2) to examine biogeographical patterns in a global sample of this species. The three loci differed significantly in mutation frequency spectra and implied population structure. Pairs of haplotypes from single populations were frequently more divergent than haplotypes sampled from widely disjunct populations. In the atpB-rbcL spacer and adk samples, Australasian haplotypes were more closely related to Northern Hemisphere haplotypes than to haplotypes found in the equatorial regions. In contrast, the phy2 sample showed that the north and south temperate regions were genetically divergent, with the equatorial regions intermediate. Maximum-likelihood estimates (MLE) of the rates of migration between the two hemispheres were significantly different for the two nuclear genes. The frequency spectra of mutations indicated that differences in implied population structure among the three loci resulted from directional selection on the chloroplast genome and on the chromosomal segment containing adk. Collectively, these data suggest that long-distance migration within the Northern Hemisphere and Australasian regions is common (relative to the mutation rate) and that migration between these two regions, potentially via equatorial populations, is more frequent than migration among equatorial populations.     

Cytoskeletal Drugs and Gravity-Induced Lateral Auxin Transport in Rice Coleoptiles

Abstract: Gravity-induced events such as amyloplast sedimentation and lateral auxin transport were probed with cytoskeletal drugs in coleoptiles of rice ( Oryza sativa L.). Amyloplast sedimentation was retarded by taxol. Lateral transport of auxin (³H-indoleacetic acid) was strongly inhibited by EPC (ethyl N-phenylcarbamate), but only partially inhibited by taxol. 1 mM EPC reduced gravitropism while phototropism was not affected. The findings suggest that microtubules may transduce pressure or proximity of amyloplasts to the auxin exporter in the plasmalemma.     

Incorporation of 5-Aminolevulinic Acid in the Chlorophyll-Protein Complexes of the Moss Ceratodon purpureus

After one month of cultivation in the dark in inorganic medium the chloroplasts of protonemata of Ceratodon purpureus have larger grana than chloroplasts from light-grown cultures. Incubation of dark-grown material with ALA increases the chlorophyll content and chlorophyll a/b ratio. On polyacrylamide-gel electrophoresis, a préferential labelling of chlorophyll-protein complex I is obtained after treatment with (³H) ALA in darkness. In contrast, in light, much higher activity is found in chlorophyll-protein complex II. The free pigment zone is highly labelled in both environments.     

Agravitropic mutants of the moss Ceratodon purpureus do not complement mutants having a reversed gravitropic response

New mutants of the moss Ceratodon purpureus have been isolated, which showed abnormal gravitropic responses. The apical cells of protonemal filaments of wild-type strains respond to gravity by growing upwards and are well aligned to the gravity vector. This response only occurs in darkness. Mutants show a range of phenotypes. Some are insensitive to gravity, showing symmetrical growth, while others align to the gravity vector but orient growth downwards. A further class grows in darkness as though it were in light, showing insensitivity to gravity and continued chlorophyll synthesis. Somatic hybrids between mutants and wild-type strains and between pairs of mutants have been selected using transgenic antibiotic resistance as selective markers. Hybrids between wild-type strains and all of the mutants have a wild-type phenotype, and so all mutants therefore have recessive phenotypes. Mutants comprise three complementation groups. One group has a single member, while another has three members. The third has at least 16 members and shows a complex pattern of complementation consistent with a single gene product functioning in both orientation and alignment to gravity, as well as contributing more than one subunit to the mature product.     

Cell wall-bound ultraviolet-screening compounds explain the high ultraviolet tolerance of the Antarctic moss, Ceratodon purpureus

* Studies of ultraviolet (UV) light-induced DNA damage in three Antarctic moss species have shown Ceratodon purpureus to be the most UV tolerant, despite containing lower concentrations of methanol-soluble UV-screening compounds than the co-occurring Bryum pseudotriquetrum. * In this study, alkali extraction of cell wall-bound phenolics, combined with methanol extraction of soluble phenolics, was used to determine whether cell wall-bound UV screens explain the greater UV tolerance of C. purpureus. * The combined pool of UV screens was similar in B. pseudotriquetrum and C. purpureus, but whilst B. pseudotriquetrum had almost equal concentrations of MeOH-soluble and alkali-extractable cell wall-bound UV-screening compounds, in C. purpureus the concentration of cell wall-bound screening compounds was six times higher than the concentration of MeOH-soluble UV screens. The Antarctic endemic Schistidium antarctici possessed half the combined pool of UV screens of the other species but, as in C. purpureus, these were predominantly cell wall bound. Confocal microscopy confirmed the localization of UV screens in each species. * Greater investment in cell wall-bound UV screens offers C. purpureus a more spatially uniform, and potentially more effective, UV screen. Schistidium antarctici has the lowest UV-screening potential, indicating that this species may be disadvantaged under continuing springtime ozone depletion. Cell wall compounds have not previously been quantified in bryophytes but may be an important component of the UV defences of lower plants.     

Identification of an ABCB/P-glycoprotein-specific Inhibitor of Auxin Transport by Chemical Genomics

Plant development and physiology are widely determined by the polar transport of the signaling molecule auxin. This process is controlled on the cellular efflux level catalyzed by members of the PIN (pin-formed) and ABCB (ATP-binding cassette protein subfamily B)/P-glycoprotein family that can function independently and coordinately. In this study, we have identified by means of chemical genomics a novel auxin transport inhibitor (ATI), BUM (2-[4-(diethylamino)-2-hydroxybenzoyl]benzoic acid), that efficiently blocks auxin-regulated plant physiology and development. In many respects, BUM resembles the functionality of the diagnostic ATI, 1-N-naphtylphtalamic acid (NPA), but it has an IC₅₀ value that is roughly a factor 30 lower. Physiological analysis and binding assays identified ABCBs, primarily ABCB1, as key targets of BUM and NPA, whereas PIN proteins are apparently not directly affected. BUM is complementary to NPA by having distinct ABCB target spectra and impacts on basipetal polar auxin transport in the shoot and root. In comparison with the recently identified ATI, gravacin, it lacks interference with ABCB membrane trafficking. Individual modes or targets of action compared with NPA are reflected by apically shifted root influx maxima that might be the result of altered BUM binding preferences or affinities to the ABCB nucleotide binding folds. This qualifies BUM as a valuable tool for auxin research, allowing differentiation between ABCB- and PIN-mediated efflux systems. Besides its obvious application as a powerful weed herbicide, BUM is a bona fide human ABCB inhibitor with the potential to restrict multidrug resistance during chemotherapy.     

Inhibition of Auxin Polar Transport Affecting the Model of Leaf Growth and Development

Tobacco (Nicotiana tabacum L. cv. Gexin No. 1) leaf slices were cultured in MS medium with different concentrations of auxin polar transport inhibitors (2, 3, 5-triiodobenzoic acid (TIBA), trans-cinnamic acid (CA), and 9-hydooxyflurence-9-carboxylic acid (HFCA)) and their effects on bud formation were observed. Although the effective concentrations vary with different inhibitors, all of them induced the formation of trumpet-shaped leaves. The frequencies of trumpet-shaped leaves were increased with the concentrations of inhibitors in media, and it was up to 82.1% when cultured in the medium containing 7.5 mg/L TIBA. The trumpet-shaped leaves were formed in different sites of the adventitious buds. These results indicated that inhibition of auxin polar transport could affect the morphogenesis of leaves, so the polar transport of auxin is essential for the bilateral symmetry of leaf growth.     

Molecular cloning of a novel phytochrome gene of the moss Ceratodon purpureus which encodes a putative light-regulated protein kinase

The phytochrome gene (phyCer) of the moss Ceratodon purpureus was isolated and characterized. phyCer is composed of three coding exons: exon I of 2035 bp, exon II of 300 bp and exon III of 1574 bp. The deduced polypeptide encoded by exon I and II exhibits substantial sequence homology to the conserved NH₂-terminal chromophore domain of known phytochromes. In contrast, the COOH-terminal polypeptide encoded by exon III shows no sequence homology to any phytochrome molecule. phyCer most likely represents a single-copy gene and is expressed in a light-independent manner. From the DNA sequence analysis it can be deduced that the PhyCer polypeptide is composed of 1303 amino acids (including the starting Met) which predicts a molecular mass for PhyCer of 145 kDa. The polypeptide encoded in exon III exhibits striking homology within the 300 carboxy-terminal amino acids to the catalytic domain of protein kinases. The carboxy terminus of PhyCer was found to be most homologous to protein-tyrosine kinases of Dictyostelium discoideum and to the products of retroviral oncogenes which belong to the Raf-Mos serine/threonine kinase family. From the hydropathy profile PhyCer appears to be a soluble protein. The predicted structure suggests that PhyCer represents a soluble light-sensor protein kinase which is linked with a cellular phosphorylating cascade.