THE 5'-CODING REGION OF PARAMECIUM SURFACE-ANTIGEN GENES CONTROLS MUTUALLY EXCLUSIVE TRANSCRIPTION

Paramecium tetraurelia stock 51 can express at least 11 different type s of surface antigens, yet only a single type is expressed on the surf ace of an individual cell at any one time, The differential expression of stock 51 type A and B surface antigen genes (51A and 51B) is regul ated at the level of transcription, Previously, we reported that nucle otide sequences upstream of position -26 (relative to the start of tra nslation) in the 51A and 51B surface antigen genes are necessary for t ranscriptional activity but are not sufficient to direct differential transcriptional control, In this report we demonstrate that at least s ome of the critical elements necessary for differential transcription of the 51A and 51B genes lie within the 5' coding region. A hybrid gen e that contains 51B upstream sequences (-475 to +1) attached to the AT G start codon of 51A is not cotranscribed with the 51B gene. In contra st, further substitution with 51B sequences (-1647 to +885) allows the chimeric gene to be coexpressed with 51B. A different hybrid gene con taining a substitution of 51B sequence from -26 to +885 in the 51A gen e is also coexpressed with 51B, revealing that the critical elements w ithin the coding region of 51B do not require 51B upstream sequences f or their effect, Coinjection of the 51A gene with the chimeric gene th at contains 51B up to +885 showed that the same sequences that allow c oexpression with 51B prevent cotranscription with 51A. Together, these results demonstrate that a region downstream of the transcriptional s tart site between nucleotide positions +1 and +885 (relative to transl ational start) is necessary to control differential transcriptional ac tivity.