THE UPSTREAM REGION IS REQUIRED BUT NOT SUFFICIENT TO CONTROL MUTUALLY EXCLUSIVE EXPRESSION OF PARAMECIUM SURFACE-ANTIGEN GENES

Paramecium tetraurelia stock 51 can express at least 11 different surf ace antigens, yet only one type is found on the surface of a cell at a ny given time. The mechanism that controls this mutually exclusive exp ression is unknown. A previous study has shown that the 51A surface an tigen gene is regulated at the level of transcription (Gilley, D., Rud man, B. M., Preer, J. R., Jr., and Polisky, B. (1990) Mol. Cell. Biol. 10, 1538-1544). We show here that the 51B surface antigen gene is als o transcriptionally regulated, and when the 51A and 51B genes are cotr ansformed into an A(-), B- mutant, the 51A antigen is dominant at 27 d egrees C just as in wild type cells. We have utilized this cotransform ation system to experimentally determine that 273 base pairs of DNA up stream of the 51A gene is sufficient to allow the dominant expression of A, but 150 base pairs is not adequate. A hybrid gene that contains the upstream region of 51B attached to the 51A transcribed region was cotransformed with the 51B gene into the A(-), B- mutant. Despite cont aining the same upstream sequences, only the hybrid 51B/A was transcri bed at 27 degrees C. These results suggest the upstream region is requ ired but not sufficient for mutually exclusive transcriptional control .