Photosynthetic enzyme and UV-irradiation studies in the evolutionary significant microbial symbiont prochloron
1998
Sese, M.L.D. (Philippines Univ. Los Banos, College, Laguna (Philippines). Inst. of Biological Sciences)
The activity of carbonic anhydrase (CA), an enzyme involved in photosynthetic carbon fixation, was studied in Prochloron, the prokaryotic symbiont of tropical marine ascidians. Measurement revealed that Prochloron exhibits CA activity largely associated with the cell surface. The extracellular CA facilitates the supply of CO2 into the cell from HCO3-, the form of inorganic carbon common at high pH values. Inhibitor studies also revealed that Prochloron CA bears similarity in terms of I50 value to cyanobacteria and green chloroplast, a further suggestion of the symbiont's evolutionary significance. Ultraviolet (UV)-irradiation severely inhibited the photosynthetic rate of Prochloron in isolation but not when present within the host tissue, suggesting some protection by the ascidian host. This protection was brought about by the thick gelatinous surface tunic covering the ascidian colony. Analysis by high performance liquid chromatography revealed that the surface tunic, although transparent to visible light, contains several UV-absorbing substances, identified as mycosporine-like amino acids (MAAS). These MAAs were shinorine (alpha max = 334 nm), mycosporine-glycine (alpha max = 310 nm) and polythine (alpha max = 320 nm). Although isolated Prochloron cells also contained shinorine, on a protein-weight basis it was less than half of that observed in the host tunic. These results suggest that the surface tunic of the ascidian colony contain UV-absorbing compounds that may shield Prochloron from the biologically hazardous incident UV radiation penetrating the shallow, tropical marine waters
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