Nascent premessenger RNA transcripts are packaged into heterogeneous nuclear ribonucleoprotein (hnRNP) complexes containing specific nuclear proteins, the hnRNP proteins. The A and B group proteins constitute a major class of small basic proteins found in mammalian hnRNP complexes. We have previously characterized the Drosophila melanogaster Hrb98DE gene, which is alternatively spliced to encode four protein isoforms closely related to the A and B proteins. We report here that the Drosophila genome contains a family of genes related to the Hrb98DE gene. One member of the family, Hrb87F, is very homologous to Hrb98DE in both sequence and structure. The Hrb87F transcripts (1.7 and 2.2 kb) utilize two alternative polyadenylation sites, are abundant in ovaries and early embryos, and are present in lesser amounts throughout development. In one wildtype strain of Drosophila there is a naturally-occurring polymorphism in this gene due to the insertion of a 412 transposable element in the 3' untranslated region. The larger transcript is not produced in these files and thus is not required for viability. Sequence identities among the Drosophila Hrb proteins and the vertebrate A and B hnRNP proteins suggest that these proteins may form a distinct subfamily within the larger family of related RNA binding proteins.

The Drosophia Hrb98DE locus encodes proteins that are highly homologous to the mammalian Al protein, a major component of heterogeneous nuclear ribonucleoprotein (RNP) particles. The Hrb98DE locus is transcribed throughout development, with the highest transcript levels found in ovaries, early embryos, and pupae. Eight different transcripts are produced by the use of combinations of alternative promoters, exons, and splice acceptor sites; the various species are not all equally abundant. The 3'-most exon is unusual in that it is completely noncoding. These transcripts can potentially generate four protein isoforms that differ in their N-terminal 16 to 21 amino acids but are identical in the remainder of the protein, including the RNP consensus motif domain and the glycine-rich domain characteristic of the mammalian Al protein. We suggest that these sequence differences could affect the affinities of the proteins for RNA or other protein components of heterogeneous nuclear RNP complexes, leading to differences in function.Transcripts synthesized by RNA polymerase II rapidly associate with a specific set of nuclear proteins to form heterogeneous nuclear ribonucleoprotein (hnRNP) complexes (for recent reviews, see references 12, 16, and 18). These RNA-protein complexes are the substrates for the processing of pre-mRNA and its transport to the cytoplasm. Studies of complexes isolated by sucrose gradient sedimentation (2), in vivo UV cross-linking (17, 19), and immunopurification (11, 35) have identified a characteristic set of proteins that are bound to the hnRNA. One of the major components of mammalian hnRNP complexes is the Al protein, a basic, glycine-rich species. The sequence of the Al protein has been determined by partial sequence analysis of protein purified from HeLa hnRNP particles (28, 41) and by sequencing rat and human cDNA clones (8,14,41

The major nuclear ribonucleoproteins (RNPs) involved in pre-mRNA processing are classified in broad terms either as small nuclear RNPs (snRNPs), which are major participants in the splicing reaction, or heterogeneous nuclear RNPs (hnRNPs), which traditionally have been thought to function in general pre-mRNA packaging. We obtained antibodies that recognize these two classes of RNP in Drosophila melanogaster. Using a sequential immunostaining technique to compare directly the distribution of these RNPs on Drosophila polytene chromosomes, we found that the two patterns were very similar qualitatively but not quantitatively, arguing for the independent deposition of the two RNP types and supporting a role for hnRNP proteins, but not snRNPs, in general transcript packaging.Both heterogeneous nuclear ribonucleoproteins (hnRNPs; reviewed in refs. 1 and 2) and small nuclear ribonucleoproteins (snRNPs; reviewed in ref.3) are deposited cotranscriptionally on eukaryotic RNA polymerase II transcripts (4-8). Whereas the major basic hnRNP proteins have been considered traditionally to function in general pre-mRNA packaging (2, 9), they have been proposed recently to be specific splicing cofactors or to be preferentially associated with splice junction sequences (10-15). snRNPs are major participants in the splicing reaction (3) but have been implicated recently in general packaging as part of a previously assembled unitary processing complex also containing hnRNPs (5, 6). The various proposals predict different amounts and ratios of the two protein types on nuclear pre-mRNA molecules at chromosomal sites of transcription, which is the issue we have addressed by sequential immunostaining.The core hnRNP proteins (A, B, and C proteins of 32-45 kDa) were originally identified as the major proteins that are associated with newly synthesized pre-mRNA (in the form of 30-50S RNP particles) when it is extracted from nuclei (reviewed in refs. 1 and 2). This observation, together with their nuclear abundance, their ability to bind single-stranded nucleic acids regardless of sequence, and their helixdestabilizing properties, led to the notion that these core hnRNP proteins are involved in general pre-mRNA packaging, much as the histones are involved in the general packaging of DNA (1, 2). However, more recent investigations of hnRNP proteins, using in vitro splicing or in vitro RNA binding studies, have suggested that these proteins play a role in the splicing reaction (10-12), that they bind with high affinity to sequences at 3' splice sites (13,14), and that they are dependent on snRNPs for acquisition of a crosslinkable association with RNA (13). These in vitro studies have led to a reappraisal of the independent structural role of hnRNP proteins in pre-mRNA packaging towards a view that they are a few of the many required cofactors for splicing. The simplest version of this view would predict a constant stoichiometry of snRNPs and the core hnRNP proteins on pre-mRNA, in amounts that correlate with the number of splicing sign...

Chimeras between human (HM-175) and simian (AGM-27) strains of hepatitis A virus (HAV) were constructed to evaluate the effect of the 2C gene of AGM-27 on HAV replication in cell culture and virulence in tamarins (Saguinus mystax) and chimpanzees (Pan troglodytes). Kinetic studies and radioimmunofocus assays demonstrated that replacement of the 2C gene of HAV/7, a cell culture-adapted strain of HM-175, with that of AGM-27 drastically reduced the ability of the virus to replicate in cultured cells. Intragenic chimeras containing AGM-27 sequences in either the 5′ or 3′ half of the 2C gene replicated in cell culture at an intermediate level. Whereas HAV/7 is attenuated for tamarins, a chimera containing the simian virus 2C gene in the HAV/7 background was virulent in tamarins, demonstrating that the simian virus 2C gene alone can confer the phenotype of virulence to an otherwise attenuated virus. Clusters of AGM-27-specific residues near both ends of the 2C protein were required for virulence since a chimera containing AGM-27 sequences in the carboxy-terminal half of 2C was partially attenuated for tamarins while one containing AGM-27 sequences only in the amino-terminal half of 2C was even more attenuated. Chimeras containing either the entire or only the 3′ half of the simian virus 2C gene in the HAV/7 background were attenuated for chimpanzees.

Mutations which positively affect growth of hepatitis A virus in cell culture may negatively affect growth in vivo. Therefore, development of an attenuated vaccine for hepatitis A may require a careful balancing of mutations to produce a virus that will grow efficiently in cells suitable for vaccine production and still maintain a satisfactory level of attenuation in vivo. Since such a balance could be achieved most directly by genetic engineering, we are analyzing mutations that accumulated during serial passage of the HM-175 strain of hepatitis A virus in MRC-5 cell cultures in order to determine the relative importance of the mutations for growth in MRC-5 cells and for attenuation in susceptible primates. Chimeric viral genomes of the HM-175 strain were constructed from cDNA clones derived from a virulent virus and from two attenuated viruses adapted to growth in African green monkey kidney (AGMK) and MRC-5 cells, respectively. Viruses encoded by these chimeric genomes were recovered by in vitro or in vivo transfection and assessed for their ability to grow in cultured MRC-5 cells or to cause hepatitis in primates (tamarins). The only MRC-5-specific mutations that substantially increased the efficiency of growth in MRC-5 cells were a group of four mutations in the 5 noncoding (NC) region. These 5 NC mutations and a separate group of 5 NC mutations that accumulated during earlier passages of the HM-175 virus in primary AGMK cells appeared, independently and additively, to result in decreased biochemical evidence of hepatitis in tamarins. However, neither group of 5 NC mutations had a demonstrable effect on the extent of virus excretion or liver pathology in these animals.