The nonstructural protein NS1 of autonomous parvoviruses is essential for viral DNA amplification and gene expression and is also the major cytopathic effector of these viruses. NS1 acts as nickase, helicase, and ATPase and upregulates P38-driven transcription of the capsid genes. We report here the identification of a novel cellular protein that interacts with NS1 from parvovirus H-1 and which we termed SGT, for small glutamine-rich tetratricopeptide repeat (TPR)-containing protein. The cDNA encoding full-length SGT was isolated through a two-hybrid screen with, as bait, the truncated NS1dlC69 polypeptide, which lacks the C-terminal transactivation domain of NS1. Full-length NS1 and SGT interacted in the two-hybrid system and in an in vitro interaction assay. Northern blot analysis revealed one major transcript of about 2 kb that was present in all rat tissues investigated. Rat sgt cDNA coded for 314 amino acids, and the protein migrated in sodium dodecyl sulfate-polyacrylamide gel electrophoresis with an apparent molecular mass of 34 kDa. SGT could be detected in both the nucleus and the cytoplasm of rat cells, as determined by indirect immunofluorescence analysis and Western blotting of fractionated cellular extracts with an affinity-purified antiserum raised against recombinant SGT (AC1.1). In H-1 virus-infected rat and human cells, compared to mock-infected controls, differences in the migration of SGT polypeptides were revealed after Western blot analysis of total cellular extracts. Moreover, the transient expression of NS proteins was sufficient to induce SGT modification. These results show that cellular SGT, which we have identified as an NS1-interacting protein, is modified by parvovirus infection as well as NS expression.

In Rhodobacter capsulatus wild-type strains, the 23S rRNA is cleaved into [16S] and [14S] rRNA molecules.Our data show that a region predicted to form a hairpin-loop structure is removed from the 23S rRNA during this processing step. We have analyzed the processing of rRNA in the wild type and in the mutant strain Fm65, which does not cleave the 23S rRNA. In addition to the lack of 23S rRNA processing, strain Fm65 shows impeded processing of a larger 5.6-kb rRNA precursor and slow maturation of 23S and 16S rRNAs from pre-23S and pre-16S rRNA species. Similar effects have also been described previously for Escherichia coli RNase III mutants. Processing of the 5.6-kb precursor was independent of protein synthesis, while the cleavage of 23S rRNA to generate 16S and 14S rRNA required protein synthesis. We identified a DNA fragment of the wild-type R. capsulatus chromosome that conferred normal processing of 5.6-kb rRNA and 23S rRNA when it was expressed in strain Fm65.The bacterial 50S ribosomal subunit is generally composed of a 23S and a 5S rRNA molecule and ribosomal proteins. However, some bacterial species have ribosomes that do not contain an intact 23S rRNA species. In vivo fragmentation of 23S rRNA has been reported for some cyanobacteria (10), for Agrobacterium tumefaciens (38), for Bdellovibrio bacteriovorus (34), for Salmonella species (5, 43), and for the closely related bacteria Rhodobacter sphaeroides (33), Rhodobacter capsulatus (28), and Paracoccus denitrificans (29). The biological significance of 23S rRNA fragmentation is not understood.Bacterial rRNA operons are transcribed into large precursor molecules often including tRNA sequences (23). These rRNA precursors are subsequently processed into the 23S, 16S, and 5S rRNA species. In Escherichia coli, two endoribonucleases that are involved in the processing of rRNA have been identified: RNase III and RNase E. RNase III cleaves doublestranded RNA regions with little sequence specificity (36,37) and is responsible for the generation of 23S and 16S rRNA precursor molecules from the larger 30S precursor (4, 44). RNase III is also responsible for the excision of intervening sequences from the rRNA of Salmonella species, resulting in fragmentation of the 23S rRNA (5). The enzymes involved in the final maturation of 16S and 23S rRNA from pre-16S and pre-23S rRNA have not been characterized; however, these reactions are most likely catalyzed by exoribonucleases (23,39). 5S rRNA is generated by cleavage of a 9S rRNA precursor that is catalyzed by the endoribonuclease RNase E (35). From comparison of different RNase E cleavage sites, the consensus sequence RAUUW (R = A or G; W = A or U) (14) has been postulated. RNase E cleaves single-stranded RNA regions with the maximal rate, when the cleavage site is preceded or followed by an mRNA hairpin-loop structure (14) (20), which is present in four copies on a single chromosome (15). The organization of the R. sphaeroides rrn operons is identical to the organization found in E. coli, with two tRNA genes in the 16...

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