Ref ID : 4392
Deborah L. Smith and F. Paul Doerder; Exceptions to Mutual Exclusion among Cell Surface i-Antigens of Tetrahymena thermophila during Salt Stress and Stationary Phase. J.Protozool. 39(5):628-635, 1992
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In ciliates, only one of the alternative forms of the immunodominant membrane glycoprotein usually coats the external surface of the cell. Such mutual exclusion is regulated at the pretranslational level by mechanisms that result in the expression of a single protein gene. In the holotrich Tetrahymena thermophila five alternative level surface immobilization proteins (i-antigens) are expressed under different conditions of temperature (L, H, T) and culture media (I, S). Using polyclonal and monoclonal antibodies to these proteins and a cDNA probe derived from the SerH3 gene, we have reinvestigated expression of i-antigens in media supplemented with 0.2M NaCl. We find that in addition to S, the H and L antigens are also present on the cell surface. While all three i-antigens may be simultaneously present on the cell surface, the combinations S/L and S/H are more frequent. Compared to cells expressing H and L singly, the level of H3 mRNA is diminished, and a subset of the L family of polypeptides is variably expressed. The expression of S begins within 30 min after transfer to NaCl-supplemented medium, while the expression of L begins three days to several weeks after transfer. When cells are transferred out of NaCl-supplemented medium, S is turned off within 24 hr, and L is expressed for at least 1 wk prior to the return of full H expression. Although these differences in kinetics suggest differences in control mechanism(s), the absence of I and T on the surface on NaCl-grown cells suggests that there is also a common regulatory link among H, S, and L. We also find that S is coexpressed with H in stationary phase cells grown in non-NaCl supplemented peptone medium. Such grown cycle dependent expression is consistent with the rapid switch in S expression upon media shift and occurs at all temperatures. A model to explain these results is presented.