| dc.description | The red alga Porphyra purpurea has a life cycle that alternates between shell-boring, filamentous sporophytes and free-living, foliose gametophytes. The significant differences between these two phases suggest that many genes should be developmentally regulated and expressed in a phase-specific manner. In this thesis, I prepared and screened subtracted cDNA libraries specific for the sporophyte and gametophyte of P. purpurea. This involved the construction of cDNA libraries, followed by the removal of common clones through subtractive hybridization. Of twenty putative phase-specific cDNAs selected from each subtracted library for further study, eight unique clones were obtained for the sporophyte and seven for the gametophyte. After confirming their phase-specificities by northern hybridizations, these 15 phase-specific cDNAs were sequenced and the deduced amino acid sequences were used to search protein data banks. The proteins encoded by two cDNAs from each phase were identified. One sporophyte-specific cDNA encodes an elongation factor 1$\alpha$(EF-1$\alpha$) normally involved in translation. Using this cDNA as a probe, the constitutively expressed EF-1$\alpha$ encoding gene (tef) was also isolated and sequenced. My results indicated that these are the only tefs in P. purpurea. The constitutively expressed gene encodes an EF-1$\alpha$ very similar to those of most eukaryotes; however, the sporophyte-specific EF-1$\alpha$ is one of the most divergent yet described. The other cDNA from the sporophyte encodes a serine protease-like protein that is structurally similar to mammalian tryptases that bind to sulphated glycosaminoglycans, suggesting that this protein may bind to sulphated galactans in the cell wall. One gametophyte-specific cDNA encodes a lipoxygenase for fatty acid metabolism and the other one encodes a protein containing repeated regions with strong similarity to fungal cellulose-binding domains. The results of in vitro translation studies of the latter cDNA suggest that, in vivo, the encoded protein enters the secretory system of the cell and may be a cell wall constituent that binds specific polysaccharides. | en_US |
