AN ALTERNATIVE PIT-1 RNA SPLICING PRODUCT REVEALS MODULAR BINDING ANDNONMODULAR TRANSCRIPTIONAL ACTIVITIES OF THE POU-SPECIFIC DOMAIN

We have identified a form of the pituitary-specific POU protein Pit-1 that results from deletion of the POU-specific (POUs) domain by altern ative RNA splicing. This natural variant of Pit-1 (called Delta 4Pit-1 ) has revealed several aspects of the function of the POU, domain. The Delta 4Pit-1 protein was characterized using a Delta 4Pit-1-specific antiserum. Further, selection assays of random oligonucleotide pools i dentified binding site preferences for both wild type and Delta 4Pit-1 . Methylation interference, copper phenanthrolene, and missing contact analyses were used to compare the binding characteristics of the two forms of Pit-1 on a selected site. DNA binding affinity assays on seve ral DNA elements revealed that the POU, domain contains a modular DNA binding activity affecting the DNA binding affinity of the entire POU domain on some, but not on other, DNA sites. Functional analysis on su ch DNA elements has revealed that the POUs domain is an essential, but nonmodular, component of the Pit-1 trans-activation domain dependent on its natural context within the Pit-1 protein.