Characterization of two nonallelic pairs of late histone H2A and H2B genes of the sea urchin: differential regulation in the embryo and tissue-specific expression in the adult.

Two nonallelic pairs of late H2A and H2B genes of the sea urchin Psammechinus miliaris were isolated on two different cosmid clones. The genes of cosmid PmL1 are separated by 11 kilobases of DNA and code for the late H2A-2 and H2B-2 variants. The genes of clone PmL2 are divergently transcribed with 1,060 base pairs of intergenic spacer DNA and code for novel variants of the H2A-2 and H2B-2 type. A comparison of the promoter sequences revealed little homology upstream of the TATA box with the exception of a 24-base-pair-long conserved sequence which is present at the same position in both late H2B promoters and part of which is identical with the "H2B-specific" 5' element. The mRNAs of the H2A and H2B genes of cosmid PmL1 reach their maximal levels early in the mesenchyme blastula embryo, whereas the transcripts of both genes of clone PmL2 accumulate maximally only later in the pluteus larva. In the adult sea urchin all four mRNAs are present in the tube foot but not in the intestine and lantern muscle. This pattern of differential expression in the embryo and tissue-specific expression in the adult suggests cell lineage-specific regulation of the late H2A-2 and H2B-2 genes. Another class of late histone genes represented by the H2A-3 and H2B-1 genes was shown to be expressed in all three adult tissues tested, whereas transcripts of the late H2A-1 genes could not be detected, suggesting that these genes are active exclusively during sea urchin development.

Octamer transcription factors and the cell type-specificity of immunoglobulin gene expression.

Antibodies are produced exclusively in B lymphocytes. The expression of the antibody-encoding genes, the immunoglobulin (Ig) genes, is also restricted to B cells. The octamer sequence ATGCAAAT is present in the promoter and the enhancer of Ig genes, and plays an important role in its tissue-specific expression. This sequence motif is a binding site for nuclear proteins, the so-called octamer transcription factors (Oct or OTF factors). The Oct-1 protein is present in all cell types analyzed so far, whereas Oct-2A and Oct-2B are found mainly in B lymphocytes. All three proteins show the same sequence specificity and binding affinity. It appears that the B cell-specific expression of Ig genes is mediated at least in part by cell type-specific Oct factors, and that there are both quantitative and qualitative differences between Oct-1 and Oct-2 factors. Recently, a number of other octamer factor variants were identified. Many of these may be created by alternative splicing of a primary transcript of one Oct factor gene and may serve a specific function in the fine tuning of gene expression.

Role of IkappaBalpha and IkappaBbeta in the biphasic nuclear translocation of NF-kappaB in TNFalpha-stimulated astrocytes and in neuroblastoma cells.

In infectious diseases of the central nervous system astrocytes respond to inflammatory cytokines like tumor necrosis factor alpha (TNFalpha) by activation of the transcription factor NF-kappaB, mediated by the proteolysis of its inhibitors IkappaBalpha and IkappaBbeta. We studied the kinetics of NF-kappaB induction by TNFalpha in primary astrocytes, and in the neuroblastoma cell line Neuro2A, and compared it to fibroblasts. In the latter, NF-kappaB DNA binding activity was induced at 30 min and remained constant up to 4 h. In contrast, in astrocytes and in Neuro2A cells NF-kappaB DNA binding activity followed a biphasic pattern: it was induced after 30 min (early phase), declined after 1 h, and increased again at 2 to 4 h (late phase). The early phase was due to rapid degradation of IkappaBalpha. After 1 h IkappaBalpha was resynthesized to levels exceeding the amounts present in unstimulated cells. This paralleled the low levels of nuclear NF-kappaB binding activity. The decrease was not observed when IkappaBalpha resynthesis was inhibited by cycloheximide. Degradation of both IkappaBalpha and IkappaBbeta contributed to the late phase of induction. However, the second peak occurred also in the absence of IkappaBbeta proteolysis, demonstrating the importance of IkappaBalpha in the formation of the biphasic nuclear translocation of NF-kappaB.

Hyperphosphorylation of mutant influenza virus matrix protein, M1, causes its retention in the nucleus.

The matrix (M1) protein of influenza virus is a major structural component, involved in regulation of viral ribonucleoprotein transport into and out of the nucleus. Early in infection, M1 is distributed in the nucleus, whereas later, it is localized predominantly in the cytoplasm. Using immunofluorescence microscopy and the influenza virus mutant ts51, we found that at the nonpermissive temperature M1 was retained in the nucleus, even at late times after infection. In contrast, the viral nucleoprotein (NP), after a temporary retention in the nucleus, was distributed in the cytoplasm. Therefore, mutant M1 supported the release of the viral ribonucleoproteins from the nucleus, but not the formation of infectious virions. The point mutation in the ts51 M1 gene was predicted to encode an additional phosphorylation site. We observed a substantial increase in the incorporation of 32Pi into M1 at the nonpermissive temperature. The critical role of this phosphorylation site was demonstrated by using H89, a protein kinase inhibitor; it inhibited the expression of the mutant phenotype, as judged by M1 distribution in the cell. Immunofluorescence analysis of ts51-infected cells after treatment with H89 showed a wild-type phenotype. In summary, the data indicated that the ts51 M1 protein was hyperphosphorylated at the nonpermissive temperature and that this phosphorylation was responsible for its aberrant nuclear retention.

Nuclear import of microinjected influenza virus ribonucleoproteins.

Influenza virus ribonucleoproteins (vRNPs) devoid of the matrix protein (M1) were isolated and introduced into the cytoplasm of CHO and MDCK cells by microinjection. The injected vRNPs were found to be imported into the nucleus, and the RNA was transcribed. Their uptake into the nucleus was ATP-dependent, inhibited by antibodies to the nuclear pore complex, unaffected by the prior acidification of the vRNPs, and not inhibited by amantadine. The results showed that for productive infection, all the early stages of the viral entry pathway (receptor interaction, endocytosis, acid exposure, and membrane fusion) can be bypassed. Once the vRNPs are stripped of M1 and separated from each other, they are competent for import into the nucleus by constitutive cellular processes. Second, the results showed that while the amantadine block for incoming virus is manifested at the level of nuclear entry of the vRNPs, the actual import event per se is not affected. The results are consistent with a recent hypothesis that amantadine inhibits a step needed to prime the core for uncoating, which takes place before the virus has reached the cytosol.

Promoters with the octamer DNA motif (ATGCAAAT) can be ubiquitous or cell type-specific depending on binding affinity of the octamer site and Oct-factor concentration.

Immunoglobulin (Ig) gene promoters contain the octamer sequence motif ATGCAAAT which is recognized by cellular transcription factors (Oct factors). Besides the ubiquitous Oct-1 factor, there is also a group of related factors (Oct-2 factors) encoded by a separate gene. The Oct-2 gene is regulated in a cell-type specific manner, and the protein is present in large amounts in B lymphocytes. We have previously shown that simple composite promoters of an octamer/TATA box type are poorly active in non-B cells but are strongly responsive to ectopic expression of Oct-2A factor, a major representative of the lymphocyte Oct-2 factors. In the present study we have tested the activity of a number of composite promoters and natural Ig promoters, and their response to Oct-1 and Oct-2 factors. Unexpectedly, we find that octamer/TATA promoters with a high affinity octamer site direct ubiquitous expression. By contrast, promoter constructions that behave in a B cell-specific manner tend to have a weak octamer binding site. These promoters are responsive to ectopic expression of additional Oct-factor, irrespective of whether it is Oct-1 or Oct-2. Using natural Ig promoters rather than composite promoters, we find that an IgH promoter is well transcribed in non-B cells via the ubiquitous Oct-1 factor, while Ig kappa and Ig lambda light chain promoters require additional Oct factor for maximal expression. It seems therefore likely that during B cell differentiation, Ig heavy chain promoters can be activated by Oct-1, before the appearance of Oct-2 factors. Oct-2 factors then would serve to boost the expression from Ig light chain promoters, which are known to be activated only after successful heavy chain gene rearrangement.

Developmental regulation of micro-injected histone genes in sea urchin embryos.

The developmental behavior of cloned histone genes of Psammechinus miliaris was studied by injection into eggs of two related sea urchin species followed by fertilization. All five early histone genes were faithfully expressed in early blastula embryos as shown by SP6 RNA mapping. A 5-10 times lower expression rate was estimated for the injected early H2A gene from its competition strength with the endogenous gene. Transcripts of this early H2A gene accumulated during the cleavage stages and decayed in late embryos in parallel with the endogenous early H2A mRNA. However, an introduced late H2B gene was incorrectly regulated, since its mRNA level did not increase from the blastula to the gastrula stage. The sperm H2B-1 gene, normally inactive in development, was 80 times less well expressed than the early H2A gene in transformed blastulae. A fusion gene with the early H2A promoter linked to the structural sperm H2B gene was, however, efficiently transcribed suggesting that all essential information for an early expression pattern is contained within the 5' region of the early H2A gene.

Astrocytes and glioblastoma cells express novel octamer-DNA binding proteins distinct from the ubiquitous Oct-1 and B cell type Oct-2 proteins.

The 'octamer' sequence, ATGCAAAT or its complement ATTTGCAT, is a key element for the transcriptional regulation of immunoglobulin genes in B-lymphocytes as well as a number of housekeeping genes in all cell types. In lymphocytes, the octamer-binding protein Oct-2A and variants thereof are thought to contribute to the B-cell specific gene expression, while the ubiquitous protein Oct-1 seems to control general octamer site-dependent transcription. Various other genes, for example interleukin-1 and MHC class II genes, contain an octamer sequence in the promoter and are expressed in cells of both the immune and nervous systems. This prompted us to analyze the octamer-binding proteins in the latter cells. Using the electrophoretic mobility shift assay, at least six novel octamer binding proteins were detected in nuclear extracts of cultured mouse astrocytes. These proteins are differentially expressed in human glioblastoma and neuroblastoma cell lines. The nervous system-derived (N-Oct) proteins bound to the octamer DNA sequence in a manner which is indistinguishable from the Oct-1 and Oct-2A proteins. The relationship of the N-Oct proteins to Oct-1 and Oct-2A was analyzed by proteolytic clipping bandshift assays and by their reactivity towards antisera raised against recombinant Oct-1 and Oct-2A proteins. On the basis of these assays, all N-Oct-factors were found to be distinct from the ubiquitous Oct-1 and the lymphoid-specific Oct-2A proteins. In melanoma cells that contain the N-Oct-3 factor, a transfected lymphocyte-specific promoter was neither activated nor was it repressed upon contransfection with an Oct-2A expression vector. We therefore speculate that N-Oct-3 and other N-Oct factors have a specific role in gene expression in cells of the nervous system.

Developmental and tissue-specific regulation of a novel transcription factor of the sea urchin.

We have identified a novel transcription factor that interacts with the promoter of four tissue-specific late histone H2A-2 and H2B-2 genes of the sea urchin by DNase I footprint, mobility shift, and methylation interference analyses. The binding site for this factor is required for efficient transcription of the H2B-2.1 gene both in vitro in nuclear extracts of gastrula embryos and in vivo in microinjected sea urchin embryos. This factor binds with equal affinity to the recognition sequences of all four histone genes in cross-competition assays. Moreover, the binding site of the H2B-2.2 promoter can functionally substitute for that of the H2B-2.1 gene in in vivo expression experiments. Nevertheless, all four binding sites share little sequence homology with each other. This transcription factor increases in abundance during embryogenesis and has been detected in the adult sea urchin only in the tube feet, where the late H2A-2 and H2B-2 genes are expressed specifically. Therefore, we refer to this factor as tissue-specific activator protein (TSAP). The close correlation between the presence of TSAP and the expression pattern of the late H2A-2 and H2B-2 genes suggests that this transcription factor is directly responsible for the developmental and tissue-specific regulation of these genes.

Octamer transcription factors bind to two different sequence motifs of the immunoglobulin heavy chain promoter.

All promoters of immunoglobulin heavy chain genes contain three conserved sequence motifs: a heptamer motif CTCATGA, an octamer motif ATGCAAAT, and a TATA box. We show that, despite their different sequences, both the heptamer and the octamer motif are bound by the same octamer transcription factors (Oct factors, also referred to as OTFs), namely the lymphoid-specific proteins Oct-2A and Oct-2B, as well as the ubiquitous protein Oct-1. Even though binding to the octamer motif is stronger, a single heptamer motif can bind Oct proteins and mediate transcriptional activity in lymphoid cells. Furthermore, factor binding to the octamer motif facilitates binding to the nearby heptamer motif. We propose that the heptamer element plays a role early in B-cell differentiation to ensure that the heavy chain promoters are transcriptionally activated before the light chain promoters, which do not contain the heptamer motif.

Puromycin-sensitive aminopeptidase. Sequence analysis, expression, and functional characterization.

Among the molecular mechanisms that control the cell division cycle, proteolysis has emerged as a key regulatory process enabling cells to pass critical check points. Such proteolysis involves a cascade of enzymes including a multisubunit complex termed 26S proteasome. Here we report on the analysis of a novel mouse cDNA encoding the puromycin-sensitive aminopeptidase (PSA) and on its expression in COS cells and 3T3 fibroblasts. PSA is 27-40% homologous to several known Zn(2+)-binding aminopeptidases including aminopeptidase N. Immunohistochemical analysis revealed that PSA is localized to the cytoplasm and to the nucleus and associates with microtubules of the spindle apparatus during mitosis. Furthermore, puromycin and bestatin both arrested the cell cycle, leading to an accumulation of cells in G2/M phase, and ultimately induced cells to undergo apoptosis at concentrations that inhibit PSA. Control experiments including cycloheximide further suggested that the induction of apoptosis by puromycin was not attributable to inhibition of protein synthesis. Taken together, these data favor the novel idea that PSA participates in proteolytic events essential for cell growth and viability.