REPRESSION OF TRANSCRIPTIONAL ACTIVITY AT A DISTANCE BY THE EVOLUTIONARILY CONSERVED KRAB DOMAIN PRESENT IN A SUBFAMILY OF ZINC-FINGER PROTEINS

Sub-families of related zinc finger protein genes have been defined on the basis of evolutionarily conserved structural features found outside the C-2-H-2 finger repeats. Such elements include the FAX domain found in a large number of Xenopus ZFPs, the evolutionarily conserved KRAB (Kruppel-associated box) and the ZiN (zinc finger N-terminal) domains. Here we describe a new evolutionarily conserved motif within zinc finger proteins which we have named the leucine rich region (LeR). Since conserved modules in regulatory proteins may specify properties relevant to their action we have determined the functional capabilities of LeR and the KRAB domains in the regulation of gene transcription by fusing relevant regions to a heterologous DNA-binding domain (GAL4 DNA-binding domain). We found that the KRAB-A domain tethered to RNA polymerase II promoters by a GAL4 DNA-binding domain actively represses transcription in a distance-independent manner. KRAB-mediated repression is dependent on the dose of the GAL4 - KRAB-A fusion protein and on the presence of GAL4 binding sites on the DNA. Conversely, the LeR domain did not modulate significantly the transcription. Our results indicate that the KRAB domain present in the non-finger region of many ZFP genes quenches transcription possibly due to specific protein - protein interactions between the KRAB-A domain and components of the proximal transcriptional apparatus.