The 120-kDa rickettsial outer membrane protein (rOmpB) is encoded by a gene with the capacity to encode a protein of approximately 168 kDa. The carboxy-terminal end of the molecule is apparently cleaved to yield 120and 32-kDa products. Both polypeptides are surface exposed and remain associated with the outer membrane of intact rickettsiae. All species of rickettsiae examined display similar cleavage of rOmpB. Comparison of diverse species of rickettsiae demonstrate a conserved N terminus of the 32-kDa fragment, with a predicted procaryotic secretory signal peptide immediately upstream of the proposed cleavage site. Coprecipitation of the 120-kDa rOmpB protein and the 32-kDa peptide by monoclonal antibodies specific for the 120-kDa portion of the molecule suggests that the two fragments remain noncovalently associated on the surface of rickettsiae. Analysis of an avirulent mutant of Rickettsia rickettsii revealed reduced amounts of the 120and 32-kDa fragments, but with a correspondingly larger rOmpB protein that displayed properties expected of the putative precursor. This avirulent mutant grows intracellularly but fails to cause the lysis of infected cells that is typical of R. rickettsii. DNA sequence analysis of the region of the gene encoding the cleavage site of the avirulent strain revealed no difference from the sequence obtained from virulent R. rickeffsii. The 168-kDa putative precursor of the avirulent strain of R. rickeffsii was not extracted from the surfaceby dilute buffers, as is the 120-kDa protein of virulent R. rickefusii or R. prowazekii. These latter results suggest that the 32-kDa C-terminal region of the molecule may serve as a membrane anchor domain.

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Immunoglobulins, containing antibodies against U1-snRNP, have been prepared from a patient with systemic lupus erythematosus. After coupling these antibodies to a Sepharose matrix, U-snRNPs have been isolated and purified from rat liver nuclei by use of immunoaffinity chromatography. The resulting RNPs had the typical protein pattern of U-snRNPs and a sedimentation coefficient of 12 S. The U-snRNP preparation was associated with an endoribonuclease which required Mg2+ for optimal activity. The enzyme, with an pH optimum of 6.2, degraded only poly(U). Other single-stranded polyribo-and polydeoxyribonucleotides, tRNA, as well as double-stranded RNA and DNA were not digested. The products of a terminal digestion are (U)6-12 with 3'-OH and 5'-P termini. The possible involvement of this endoribonuclease in the splicing of hnRNA is discussed.Most eukaryotic genes, besides exon regions represented in the mRNA, also contain intron sequences. These are removed in the nucleus from the primary transcript (hnRNA) by splicing reactions [l]. It is not yet clear at which time during the overall course of post-transcriptional processing of mRNA splicing occurs [2,3]. Some evidence is available indicating that the poly(A) segment of mRNA [2] and/or nucleoplasmic U-snRNPs [4] orient the splicing of mRNA precursors. Comparison of the consensus sequences at RNA splicing sites in pre-mRNA with sequences in U1 RNA revealed an exact complementarity [5,6], suggesting a model in which U1 RNP is one recognition component of the nuclear RNA splicing apparatus. This assumption is supported by the finding that snRNAs are directly associated with hnRNA [7,8]. Moreover, it has been demonstrated that lupus antiserum, which precipitates U1 RNP, inhibits splicing of adenovirus mRNA by isolated HeLa cell nuclei [9]. Up to now, enzymes involved in mRNA splicing have not been characterized. In a recent contribution [lo] we reported that endoribonuclease VII is a candidate for a splicing enzyme in a hypothesized poly(A)-mediated splicing process. This enzyme was found to be modulated by poly(A) and is able to recognize oligo(U) sequences.Supposing that U1-snRNA is essential for the splicing reactions, it seems reasonable to search also for an enyzme(s)

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