on carbon fixation remains obscure. Work on Chlamydomonas 

 reinhardtii grown under nitrogen-limitation showed that all the 

 components for light-energy transduction become severely depleted. 

 In addition, modifications of some of the complexes could be 

 discerned, including the appearance of novel, light-harvesting 

 pigment-protein complexes and a reduction in the amount of 

 chlorophyll bound to the Photosystem I reaction center. While 

 these specific phenomena are not observed in the marine 

 phytoplankter, Isochrysis qalbana , losses of peripheral antenna 

 protein complexes occur in response to a reduction in nitrogen 

 supply. 



To identify the molecular basis for adaptation to poor growth 

 conditions, rates of photosynthetic protein synthesis and levels of 

 messenger RNAs for photosynthetic proteins were examined. These 

 analyses demonstrated that nitrogen deficiency changes the 

 expression of nuclear genes for pigment-binding proteins: certain 

 normally high, abundant mRNAs disappear while new mRNAs for 

 related, but functionally distinct, light-harvesting proteins 

 accumulate. Within chloroplasts, all mRNAs for photosynthetic 

 proteins are present at normal levels, but only a few are used to 

 control rates of protein synthesis. Together, these data indicate 

 that the availability of nitrogen can influence nuclear gene 

 transcription and translation of chloroplast mRNA. Furthermore, 

 the nitrogen-deficiency phenotype is rapidly reversed when 

 nutrients are added to the cultures. 



It is well established that nitrogen status determines the 

 stability of the photosynthetic apparatus and its response to 

 light-dependent damage (photoinhibition) (Ferrar and Osmond, 1986; 

 Henley et al., 1991). A key element in photoinhibitory damage 

 centers on the D-l protein of the rapidly turning-over PS II 

 reaction center (Kyle, 1987) and on protective mechanisms 

 associated with the xanthophyll cycle (Deming-Adams, 1990) . 

 Probing these specific reactions by molecular methods may help 

 describe large system functions (e.g., marine phytoplankton) in 

 global carbon fluxes. 



Much remains to be learned about nitrogen deficiency at the 

 molecular, cellular, and ecosystem levels. Are the genes encoding 

 the proteins in the nucleus, or, as DNA sequence data suggest, do 

 some genes reside in chloroplasts? How do cells detect nitrogen- 

 deficiency, and how does it prompt changes in the expression of 



1-4 



