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 cell 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 0.2 M 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 h, 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 of NaCl-grown cells suggests that there is also a common regulatory link 
among H, S and L.(ABSTRACT TRUNCATED AT 250 WORDS)