A structurally novel transferrin-like protein accumulates in the plasma membrane of the unicellular green alga Dunaliella salina grown in high salinities.

The alga Dunaliella salina is outstanding is its ability to withstand extremely high salinities. To uncover mechanisms underlying salt tolerance, a search was carried out for salt-induced proteins. The level of a plasma membrane 150-kDa protein, p150, was found to increase with rising external salinity (Sadka, A., Himmelhoch, S., and Zamir, A. (1991) Plant Physiol. 95, 822-831). Based on its cDNA-deduced sequence, p150 belongs to the transferrin family of proteins so far identified only in animals. This, to our best knowledge, is the first demonstration of a transferrin-like protein in a photosynthetic organism. Unlike animal transferrins, p150 contains three, rather than two, internal repeats and a COOH-terminal extension including an acidic amino acid cluster. In intact cells p150 is degraded by Pronase, indicating that the protein is extracellularly exposed. The relationship of p150 to iron uptake is supported by the induction of the protein in iron-deficient media and by its radioactive labeling in cells grown with 59Fe. Accumulation of p150 is transcriptionally regulated. It is proposed that p150 acts in iron uptake other than by receptor-mediated endocytosis and that its induction permits the cells to overcome a possible limitation in iron availability under high salinities.

A salt-resistant plasma membrane carbonic anhydrase is induced by salt in Dunaliella salina.

The mechanisms allowing proliferation of the unicellular green alga Dunaliella salina in up to saturating NaCl concentrations are only partially understood at present. Previously, the level of a plasma membrane Mr 60,000 protein, p60, was found to increase with rising external salinities. Based on cDNA cloning and enzymatic assays, it is now shown that p60 is an internally duplicated carbonic anhydrase, with each repeat homologous to animal and Chlamydomonas reinhardtii carbonic anhydrases, but exceptional in the excess of acidic over basic residues. Increasing salinities, alkaline shift, or removal of bicarbonate induced in D. salina parallel increases in the levels of p60, its mRNA, and external carbonic anhydrase activity. Moreover, purified p60 exhibited carbonic anhydrase activity comparable to other carbonic anhydrases. A p60-enriched soluble preparation showed maximal carbonic anhydrase activity at approximately 1.0 M NaCl and retained considerable activity at higher salt concentrations. In contrast, a similar preparation from C. reinhardtii was approximately 90% inhibited in 0.6 M NaCl. These results identified p60 as a structurally novel carbonic anhydrase transcriptionally regulated by CO2 availability and exhibiting halophilic-like characteristics. This enzyme is potentially suited to optimize CO2 uptake by cells growing in hypersaline media.

Regulation and light-harvesting complex II association of a Dunaliella protein homologous to early light-induced proteins in higher plants.

The unicellular green alga Dunaliella bardawil responds to high light, nutrient deprivation, and several other types of stress by massive accumulation of beta-carotene. We have previously cloned a nuclear gene, cbr, that is co-induced with accelerated carotenogenesis. The predicted product of cbr is closely related to early light-induced proteins (Elips) of higher plants, and also shows resemblance to chlorophyll a/b-binding proteins. In the present study, the determination of the cbr transcription start site supported the previously proposed site of translation initiation. Antibodies raised against a synthetic oligopeptide matching the predicted sequence of Cbr recognized two polypeptides of apparently 17 and 19 kDa that were induced in parallel to cbr transcript accumulation in highly illuminated or sulfate-starved D. bardawil cells. The antibodies also cross-reacted with an approximately 20-kDa polypeptide in sulfate-starved Dunaliella salina. In both D. bardawil and D. salina, Cbr was found exclusively in the thylakoid membranes. After mild solubilization, Cbr co-fractionated with light-harvesting complex II (LHCII) in sucrose gradient centrifugation and gel electrophoresis and was specifically associated with a minor LHCII complex. An occasionally observed, faster mobility Cbr form, free of LHCII, was probably released from the larger complex. These results support the conclusion that Cbr belongs to a class of stress-induced proteins transiently associated with antennae complexes.

Serum protein polymorphisms in the population of south Yemen.

The Gc, Hp, and Tf polymorphisms were studied in the population of South Yemen. The gene frequencies were in agreement with those of other populations in the Middle East. There was an indication of local variations due to ethnic heterogeneity, e.g. a relatively high frequency of TfD in 1 of the 5 subpopulations studied.

Characteristics of potential lymphoma-inducing cells in mice sensitive or resistant to lymphomagenesis by radiation leukemia virus variants.

The relationship between the H-2-associated responsiveness of mice to radiation leukemia virus variants (A-RadLV and D-RadLV) lymphomagenesis and the characteristics of early occurring potential lymphoma-inducing cells (PLC) among thymus and bone marrow cells of these virus-infected mice was investigated. Sensitivity to virus-induced T-cell lymphomagenesis was shown to involve early occurrence of Thy-positive PLC, found predominantly among thymocytes, whereas resistance was rather related with early identification of PLC-Thy-negative cells mostly among bone marrow cells. PLC were further characterized in the sensitive (BL/6 + A-RadLV) and resistant (BL/6 + D-RadLV) situations by testing in parallel the tumorigenic potential (using the transplantation bioassay method) and type of thymus and bone marrow cell populations separated by different methods such as size fractionation by centrifugal elutriation, cytotoxic elimination of lymphocytes, or panning. The early occurring PLC among thymocytes of BL/6 mice 10-20 days following infection with A-RadLV were shown to be cortisone-resistant, Thy+, CD4+, and/or CD8+ medium size dividing thymocytes. PLC among thymocytes of BL/6 mice + D-RadLV, identified among the medium and large cell fractions, were shown to be cortisone-resistant Thy-, CD4-CD8- lymphocytes. High tumorigenic potential of PLC was demonstrated only among unseparated or separated (on size basis) bone marrow cells of BL/6 + D-RadLV (72-84%), whereas unseparated or separated fractions of bone marrow from BL/6 + A-RadLV had a low lymphomagenic potential (15-20%). The parallelism between the bone marrow fractions that induced optimal thymus cellularity following reconstitution of lethally irradiated mice and optimal lymphomagenicity stress the prothymocyte characteristics of PLC among bone marrow cells of BL/6 mice infected with D-RadLV. It is suggested that in resistant and sensitive haplotypes RadLV variants infect different cell populations and thereby induce PLC which differ in their capacity to present associative MuLV antigens with self H-2.