Summary 1. Heterocysts are found in many species of filamentous blue‐green algae. They are cells of slightly larger size and with a more thickened wall than the vegetative cells. 2. Structural details of the heterocyst are: the presence of three additional wall layers, the absence of granules, sparse thylakoid network throughout, except at the poles where a dense coiling of membranes occurs. Other characters include the two pores at opposite poles ‘plugged’ with refractive material called the polar granule. 3. Peculiarities in the pigment composition of the heterocyst include an abundance of carotenoids and absence of phycobilins, and a short‐wave form of chlorophyll a. 4. Unique glycolipids and an acyl lipid, not found in the vegetative cells of the algae or in other plant cells, are associated with the heterocyst. The glycolipids constitute the laminated layer of the wall and probably regulate diffusion of substances through it, whereas the acyl lipids are supposed to function as carriers and intermediates in the biosynthesis of the wall. 5. The heterocysts develop from vegetative cells, and the visible changes during differentiation include cell enlargement, synthesis of additional wall layers, disappearance of granules and reorientation and synthesis of the thylakoids. 6. Heterocysts are formed sequentially with characteristic cellular spacing during the growth of cultures in medium free from combined nitrogen. 7. Various sources of combined nitrogen inhibit heterocyst formation when supplied in the culture medium. Ammonium salts are among the most powerful inhibitors. Heterocysts are formed simultaneously and within a short period after transference of ammonia‐grown non‐heterocystous filaments to ammonia‐free medium. 8. Incompletely differentiated heterocysts or proheterocysts are found in cultures grown in the presence of combined nitrogen. If two or more proheterocysts are close together generally a single one develops to maturity after a competitive interaction in medium free from combined nitrogen. This indicates that heterocyst formation is completed in two phases: phase I, synthesis and conservation of macromolecules, which takes place during growth in ammonia‐containing medium: and phase 11, morphological differentiation of the heterocyst which is unaccompanied by growth in cell number. In the ammonia‐free medium phase 11 quickly succeeds phase 1 and the whole process appears as a continuum. 9. Heterocyst formation shows a definite requirement for light. Red light favours heterocyst formation, whereas green and blue light do not. The effects of light seem to be mainly due to photosynthesis, although some effects may be morphogenetic. 10. Studies with metabolic inhibitors have revealed the involvement of photosynthesis, respiration and protein synthesis in heterocyst formation. Photosynthesis provides carbon skeletons, whereas ATP is most probably supplied by oxidative metabolism. 11. Various functions have been assigned to the heterocyst from time to time. Their role in akinete formation is suggested ...
Dihydrodipicolinic acid reductase, an enzyme which catalyzes the pyridine nucleotide-linked reduction of dihydrodipicolinic acid to tetrahydrodipicolinic acid in the biosynthetic pathway leading to L-lysine, has been partially purified from maize (Zea mays cv Pioneer 3145) kernels. The crude maize extract and the partially purified enzyme were assayed for dihydrodipicolinic acid reductase by their ability to restore the capability of crude extracts of a mutant Escherichia coli (CGSC 4549; defective in dihydrodipicolinic acid reductase) to synthesize diaminopimelic acid from aspartic acid and pyruvic acid.
Visible absorption spectra are presented for the AzoUa caroliniana Willd.-Anabaena azola Strass. association and the individual partners.Although absorption by the phycobiliproteins of the endophytic cyanobacterium clearly complements the absorption by the fern pigments, their contribution to the absorption spectrum of the association is effectively concealed by the preponderance of the Azolla pigments. Action spectra for nitrogenase-catabzed C2H2 reduction in both the AzoUa-Anabaena association and the endophytic Anabaena demonstrate that quanta absorbed by the phycobiliproteins is as effective as that absorbed by chlorophyll a in driving this photosystem I-linked process. Under anaerobic conditions, the inhibition of photosystem II activty by 3-(3,4-dichlorophenyl)-1,1-dimethylurea, diuron did not selectively decrease the relative quantum yields in the region of phycobiliprotein absorption. At the well-below saturating light intensities used for the action spectra studies, the absolute rates of C2H2 reduction were increased uniformly via respiratory-linked processes under aerobic conditions. The occurrence of phycobiliproteins in heterocysts of the endophytic Anabaena was demonstrated using fluorescence microscopy of intact filaments. Fluorescence micrographs of Anabaena cylidrica filaments are presented for comparison.Specialized cavities formed in the aerial dorsal leaf lobes of the aquatic fern Azolla are occupied by an endophytic cyanobacterium known as Anabaena azollae Strass (9). The endophyte, which undergoes a pattern of development and differentiation as a function of leaf age (5, 6), can provide the association with its total N requirement by N2 fixation.As in free-living cyanobacteria (4,18,20,21) there is a close relationship between photosynthesis and N2 fixation in the AzollaAnabaena association (10, 11). Action spectra for photosynthesis in the Azolla-Anabaena association, Azolla freed of the Anabaena, and the Anabaena removed from the fern have been presented previously (16). Although there was no readily discernible contribution by the endophyte to the action spectrum for photosynthesis in the association, the maximum yield per incident quantum for photosynthesis in the isolated endophyte was quite distinct from that of the endophyte-free Azolla Absorption Spectra of Azoffa Leaves and Endophyte. Azolla stem segments bearing leaves were mounted in a single layer between two pieces of glass. Glass spacers prevented crushing. A similar procedure was employed with the isolated endophyte except that the inserts were omitted and edges of the glass were sealed with dental wax. The glass holders were masked with black tape so that only the light beam passing through the sample was measured. To minimize the effect of scattering, the samples were positioned adjacent to the detector of a Perkin-Elmer 557 spectrophotometer.Action Spectra. Acetylene reduction was used as a measure of nitrogenase activity. For the association, sufficient plant material was added to uniformly cover the liquid surf...
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