We investigated the role of SIRT7, one of the seven members of the mammalian sirtuin family. We show that SIRT7 is a widely expressed nucleolar protein that is associated with active rRNA genes (rDNA), where it interacts with RNA polymerase I (Pol I) as well as with histones. Overexpression of SIRT7 increases Pol I-mediated transcription, whereas knockdown of SIRT7 or inhibition of the catalytic activity results in decreased association of Pol I with rDNA and a reduction of Pol I transcription. Depletion of SIRT7 stops cell proliferation and triggers apoptosis. Our findings suggest that SIRT7 is a positive regulator of Pol I transcription and is required for cell viability in mammals. Supplemental material is available at http://www.genesdev.org.

Members of the Sir2 family of NAD-dependent protein deacetylases regulate diverse cellular processes including aging, gene silencing, and cellular differentiation. Here, we report that the distant mammalian Sir2 homolog SIRT6 is a broadly expressed, predominantly nuclear protein. Northern analysis of embryonic samples and multiple adult tissues revealed mouse SIRT6 (mSIRT6) mRNA peaks at day E11, persisting into adulthood in all eight tissues examined. At the protein level, mSIRT6 was readily detectable in the same eight tissue types, with the highest levels in muscle, brain, and heart. Subcellular localization studies using both C-and N-terminal green fluorescent protein fusion proteins showed mSIRT6-green fluorescent protein to be a predominantly nuclear protein. Indirect immunofluorescence using antibodies to two different mSIRT6 epitopes confirmed that endogenous mSIRT6 is also largely nuclear. Consistent with previous findings, we did not observe any NAD ؉ -dependent protein deacetylase activity in preparations of mSIRT6. However, purified recombinant mSIRT6 did catalyze the robust transfer of radiolabel from [ 32 P]NAD to mSIRT6. Two highly conserved residues within the catalytic core of the protein were required for this reaction. This reaction is most likely mono-ADP-ribosylation because only the modified form of the protein was recognized by an antibody specific to mono-ADP-ribose. Surprisingly, we observed that the catalytic mechanism of this reaction is intra-molecular, with individual molecules of mSIRT6 directing their own modification. These results provide the first characterization of a Sir2 protein from phylogenetic class IV.

The SSD1 gene of Saccharomyces cerevisiae is a polymorphic locus that affects diverse cellular processes including cell integrity, cell cycle progression, and growth at high temperature. We show here that the SSD1-V allele is necessary for cells to achieve extremely long life span. Furthermore, addition of SSD1-V to cells can increase longevity independently of SIR2, although SIR2 is necessary for SSD1-V cells to attain maximal life span. Past studies of yeast aging have been performed in short-lived ssd1-d strain backgrounds. We propose that SSD1-V defines a previously undescribed pathway affecting cellular longevity and suggest that future studies on longevity-promoting genes should be carried out in long-lived SSD1-V strains.

Prochlorococcus is the smallest oxygenic phototroph yet described. It numerically dominates the phytoplankton community in the mid-latitude oceanic gyres, where it has an important role in the global carbon cycle. The complete genomes of several Prochlorococcus strains have been sequenced, revealing that nearly half of the genes in each genome are of unknown function. Genetic methods, such as reporter gene assays and tagged mutagenesis, are critical to unveiling the functions of these genes. Here, we describe conditions for the transfer of plasmid DNA into Prochlorococcus strain MIT9313 by interspecific conjugation with Escherichia coli. Following conjugation, E. coli bacteria were removed from the Prochlorococcus cultures by infection with E. coli phage T7. We applied these methods to show that an RSF1010-derived plasmid will replicate in Prochlorococcus strain MIT9313. When this plasmid was modified to contain green fluorescent protein, we detected its expression in Prochlorococcus by Western blotting and cellular fluorescence. Further, we applied these conjugation methods to show that a mini-Tn5 transposon will transpose in vivo in Prochlorococcus. These genetic advances provide a basis for future genetic studies with Prochlorococcus, a microbe of ecological importance in the world's oceans.Prochlorococcus, a unicellular cyanobacterium, is globally distributed throughout the temperate ocean gyres. Densities of Prochlorococcus reach 700,000 cells ml Ϫ1 of seawater (2), likely making it the most numerically abundant photosynthetic organism in the oceans (20). As a consequence of its global abundance, Prochlorococcus plays an important role in the global carbon cycle. For example, up to 79% of primary productivity in the North Atlantic is due to Prochlorococcus (11). The Prochlorococcus community is composed of a number of related strains (16,26) that have different depth distributions in the ocean and thus occupy distinct ecological niches with respect to light and nutrients. The strain used in this study, Prochlorococcus strain MIT9313, is relatively most abundant near the base of the euphotic zone at depths around 100 m (9, 28, 30). In accordance with its depth distribution, MIT9313 grows at lower light levels than other strains (14), which are relatively more abundant in the surface waters. The nutrient physiology of MIT9313 also appears to be adapted to conditions deep in the euphotic zone. For example, MIT9313 has nitrite transport and reduction genes (21) and grows on this nitrogen source (15), whereas the high-light-level-adapted MED4 strain lacks genes for nitrite utilization (21) and does not grow on nitrite (15). The ability to grow on nitrite correlates with the depth distribution of the MIT9313 ecotype, since a well-defined nitrite maximum is often found at the base of the euphotic zone (18). Prochlorococcus strains thus have distinct nutrient physiologies that likely represent adaptations for exploitation of distinct niches in the ocean water column (reviewed in reference 7).The global abundance of...

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