Mouse ribosomal genes have a short sequence upstream of the transcription initiation site that is related in structure and function to the terminator boxes previously identified at the 3' end of the transcription unit. This upstream terminator recognizes the same protein factor as the 3'-terminal sites and is able to terminate RNA polymerase I transcription in vitro. S1 mapping and nucleolar run-on experiments reveal the presence of 5'-terminal spacer transcripts that are terminated at this site. These transcripts probably derive from spacer promoters, one of which has been identified approximately 2 kb upstream of the pre-rRNA start site. The interaction of a specific nuclear factor with the upstream terminator increases the efficiency of initiation, suggesting that transcription termination and initiation at the adjacent promoter work in an interrelated fashion.

The 18-base-pair sequence element AGGTCGACCAGTACTCCG (the Sal box) signals termination of mouse ribosomal gene transcription. This sequence is recognized by a sequence-specific DNA-binding protein, TTF I, which mediates the termination of transcription by RNA polymerase I (pol I). Subsequently, the ends of the primary transcripts are trimmed by 10 nucleotides in a sequence-dependent 3'-terminal processing reaction. We have now investigated whether TTF I bound to its target sequence will block elongation by any RNA polymerase by steric hindrance, or whether it is specific for elongation by pol I. The results demonstrate that TTF I directs transcription termination with RNA polymerase I from species as divergent as mouse and yeast, but fails to affect elongation by heterologous polymerases (eukaryotic RNA polymerases II and III, Escherichia coli or bacteriophage T3 RNA polymerase). By contrast, purified lac repressor bound to its operator sequence stops elongation by both RNA polymerase I and II.

Septins constitute a group of GTP binding proteins that assemble into homo- and hetero-oligomeric complexes and filaments. These higher order septin structures are thought to function like scaffolds and/or diffusion barriers serving as spatial localizers for many proteins with key roles in cell polarity and cell cycle progression. In this study, we extensively characterized septin interaction partners using yeast two-hybrid and three-hybrid systems in addition to precipitation analyses in platelets. As a result, we identified human hetero-trimeric septin complexes on a large scale, which had been only postulated in the past. In addition, we illustrated roles of SEPT9 that might contribute to hetero-trimeric septin complex formation. SEPT9 can substitute for septins of the SEPT2 group and partially for SEPT7. Mutagenic analyses revealed that mutation of a potential phosphorylation site in SEPT7 (Y318) regulates the interaction with other septins. We identified several septin-septin interactions in platelets suggesting a regulatory role of diverse septin complexes in platelet function.

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