genes for chlororespiration and photosynthetic proteins? Is 

 chlororespiration truly necessary for survival of photosynthetic 

 organisms in nitrogen-poor environments? Does chlororespiration 

 develop in all nutrient-deficient photosynthetic organisms, or are 

 there alternative strategies for survival? 



Iron Limitation 



Some regions of the oceans have high levels of dissolved 

 inorganic phosphorus and nitrogen, and it has been hypothesized 

 that iron may limit primary productivity there. As a result, 

 laboratory studies have been initiated to promote understanding of 

 iron-limitation at the molecular level. 



Cyanobacteria can live in environments that differ widely in 

 the amount of available iron. When iron is limited, the cells grow 

 well, but the biomass is lowered and cellular pigmentation is 

 substantially altered, as is the case for nitrogen limitation. 

 Phycobilisomes are no longer formed and the chlorophyll-protein 

 composition is changed. These alterations can be visualized by 

 changes in the absorption spectrum and in the fluorescence 

 spectrum. Among other events is synthesis of a new chl-protein 

 (CP) complex, CP43 1 , which is similar to a normal PSII CP complex, 

 CP43, except that it lacks a 100 amino-acid domain localized in the 

 membrane lumen. This modified protein is synthesized only under 

 iron efficiency and can be diagnostic of this condition. The gene 

 is controlled by an iron-regulated promoter, which appears to be 

 recognized specifically when iron is deficient. A similar 

 phenomenon is found in the marine diatom, Phaedactvlum tricornutum . 



This system provides a wealth of probes that can be used to 

 analyze iron deficiency in natural populations, including spectral 

 analyses, (a) absorption spectra — loss of absorption at 625 nm 

 (phycocyanin) and Chi shift to 675 nm, and (b) 77°K fluorescence 

 spectra — > total loss of 696 nm and 716 nm fluorescence, and a 

 large increase in 685 nm fluorescence (due to new CP complex) . The 

 antibody against a portion of the new protein (CP43') is highly 

 specific and can be used as a probe for detecting iron limitation. 

 A DNA probe can detect the expression of gene under iron-deficient 

 conditions. A promoter can be cloned in front of the reporter 

 gene, whose expression will be turned on only under iron 

 deficiency. If a genetic transformation system is available, such 



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