OLIGOMERIZATION OF THE TRANSCRIPTION TERMINATION FACTOR TTF-I - IMPLICATIONS FOR THE STRUCTURAL ORGANIZATION OF RIBOSOMAL TRANSCRIPTION UNITS

Mammalian ribosomal genes are flanked at. their 5' and 3' ends by term inator sequences which are recognized by the transcription termination factor TTF-l. The occurrence of the same binding site upstream and do wnstream of the gene raises the possibility that TIF-I can interact wi th both sequences simultaneously and thus brings the terminator in the vicinity of the gene promoter by looping out the pre-rRNA coding sequ ence, To test this model, we have examined the ability of TTF-I to oli gomerize and found that both full-length and N-terminally truncated ve rsions of TTF-I form stable oligomeric structures, At least two domain s of TTF-I located within the 184 N-terminal and 445 C-terminal amino acids, respectively, mediate the self-association of several TTF-I mol ecules, In support of the looping model, TTF-l is capable of linking t wo separate DNA fragments via binding to the target sites, This result indicates that in addition to its function in transcription terminati on, TTF-l may serve a role in the structural organization of the ribos omal genes which may be important for maintaining the high loading den sity of RNA polymerase I on active rRNA genes.