Both conserved signals on mammalian histone pre-mRNAs associate with small nuclear ribonucleoproteins during 3' end formation in vitro

Genetic contribution of HIST1H1T regulatory region alternations to human nonobstructive azoospermia

HIST1H1T encodes H1T, a testicular variant of histone H1, which is expressed during spermatogenesis especially in primary spermatocytes and facilitates histone to protamine exchanges during maturation of spermatozoa. The goal of the conducted research was to evaluate four genetic variations of HIST1H1T in men with nonobstructive azoospermia. This case-control study was conducted among a total number of 200 men, including 100 nonobstructive azoospermic (NOA) infertile men. In this study, three single-nucleotide polymorphisms, including c.-54C>T (rs72834678), c.-912A>C (rs707892) and c.-947A>G (rs74293938) in regulatory region as well as one SNP c.40G>C (rs198844) in coding region were identified using PCR sequencing. According to statistical analysis, none of those SNPs in regulatory regions showed significant differences in case and control groups. For SNP (c.40G>C), a significantly higher frequency of C allele in the case group was observed compared to the control group (p-value: .044). In conclusion, according to statistical analysis it seems that the polymorphism of c.40G>C is not associated with nonobstructive azoospermia.

Expression of histone-U1 snRNA chimeric genes: U1 promoters are compatible with histone 3' end formation

Chimeric genes which fuse the mouse histone H2a gene and the mouse U1b gene were constructed and introduced into CHO cells by cotransfection. In the UH genes, the U1b gene promoter and the start of the U1b gene were fused to the H2a gene in the 5' untranslated region. In the HU genes, the U1b 3' end was inserted into the 3' untranslated region of the H2a gene replacing the normal histone 3' end. Transcripts from the UH genes initiated at the start of the U1 gene and ended at the normal histone 3' end. Transcripts from the HU chimeric genes did not end at the U1 3' end but extended at least 80 nucleotides further and had heterogeneous 3' ends. Placing both a U1 snRNA promoter and a U1 snRNA 3' end around a histone coding region resulted in transcripts which initiate and terminate at the appropriate U1 ends. These results are consistent with previous reports that formation of the U1 3' ends require U1 promoters, but indicate that the histone 3' end can be formed on transcripts initiating at U1 promoters. The transcripts initiated at the U1 start site and ending at the histone 3' end are present on polyribosomes and show proper posttranscriptional regulation.

U7 snRNP is recruited to histone pre-mRNA in a FLASH-dependent manner by two separate regions of the stem-loop binding protein

Cleavage of histone pre-mRNAs at the 3' end requires stem-loop binding protein (SLBP) and U7 snRNP that consists of U7 snRNA and a unique Sm ring containing two U7-specific proteins: Lsm10 and Lsm11. Lsm11 interacts with FLASH and together they bring a subset of polyadenylation factors to U7 snRNP, including the CPSF73 endonuclease that cleaves histone pre-mRNA. SLBP binds to a conserved stem-loop structure upstream of the cleavage site and acts by promoting an interaction between the U7 snRNP and a sequence element located downstream from the cleavage site. We show that both human and Drosophila SLBPs stabilize U7 snRNP on histone pre-mRNA via two regions that are not directly involved in recognizing the stem-loop structure: helix B of the RNA binding domain and the C-terminal region that follows the RNA binding domain. Stabilization of U7 snRNP binding to histone pre-mRNA by SLBP requires FLASH but not the polyadenylation factors. Thus, FLASH plays two roles in 3' end processing of histone pre-mRNAs: It interacts with Lsm11 to form a docking platform for the polyadenylation factors, and it cooperates with SLBP to recruit U7 snRNP to histone pre-mRNA.

The histone genes cluster in Rhynchosciara americana and its transcription profile in salivary glands during larval development

In this work we report the characterization of the Rhynchosciara americana histone genes cluster nucleotide sequence. It spans 5,131 bp and contains the four core histones and the linker histone H1. Putative control elements were detected. We also determined the copy number of the tandem repeat unit through quantitative PCR, as well as the unequivocal chromosome location of this unique locus in chromosome A band 13. The data were compared with histone clusters from the genus Drosophila, which are the closest known homologues.

Degradation of oligouridylated histone mRNAs: see UUUUU and goodbye

During the cell cycle the expression of replication-dependent histones is tightly coupled to DNA synthesis. Histone messenger RNA (mRNA) levels strongly increase during early S-phase and rapidly decrease at the end of it. Here, we review the degradation of replication-dependent histone mRNAs, a paradigm of post-transcriptional gene regulation, in the context of processing, translation, and oligouridylation. Replication-dependent histone transcripts are characterized by the absence of introns and by the presence of a stem-loop structure at the 3' end of a very short 3' untranslated region (UTR). These features, together with a need for active translation, are a prerequisite for their rapid decay. The degradation is induced by 3' end additions of untemplated uridines, performed by terminal uridyl transferases. Such 3' oligouridylated transcripts are preferentially bound by the heteroheptameric LSM1-7 complex, which also interacts with the 3'→5' exonuclease ERI1 (also called 3'hExo). Presumably in cooperation with LSM1-7 and aided by the helicase UPF1, ERI1 degrades through the stem-loop of oligouridylated histone mRNAs in repeated rounds of partial degradation and reoligouridylation. Although histone mRNA decay is now known in some detail, important questions remain open: How is ceasing nuclear DNA replication relayed to the cytoplasmic histone mRNA degradation? Why is translation important for this process? Recent research on factors such as SLIP1, DBP5, eIF3, CTIF, CBP80/20, and ERI1 has provided new insights into the 3' end formation, the nuclear export, and the translation of histone mRNAs. We discuss how these results fit with the preparation of histone mRNAs for degradation, which starts as early as these transcripts are generated.

The Drosophila U7 snRNP proteins Lsm10 and Lsm11 are required for histone pre-mRNA processing and play an essential role in development

Metazoan replication-dependent histone mRNAs are not polyadenylated, and instead terminate in a conserved stem-loop structure generated by an endonucleolytic cleavage of the pre-mRNA involving U7 snRNP. U7 snRNP contains two like-Sm proteins, Lsm10 and Lsm11, which replace SmD1 and SmD2 in the canonical heptameric Sm protein ring that binds spliceosomal snRNAs. Here we show that mutations in either the Drosophila Lsm10 or the Lsm11 gene disrupt normal histone pre-mRNA processing, resulting in production of poly(A)+ histone mRNA as a result of transcriptional read-through to cryptic polyadenylation sites present downstream from each histone gene. This molecular phenotype is indistinguishable from that which we previously described for mutations in U7 snRNA. Lsm10 protein fails to accumulate in Lsm11 mutants, suggesting that a pool of Lsm10-Lsm11 dimers provides precursors for U7 snRNP assembly. Unexpectedly, U7 snRNA was detected in Lsm11 and Lsm1 mutants and could be precipitated with anti-trimethylguanosine antibodies, suggesting that it assembles into a snRNP particle in the absence of Lsm10 and Lsm11. However, this U7 snRNA could not be detected at the histone locus body, suggesting that Lsm10 and Lsm11 are necessary for U7 snRNP localization. In contrast to U7 snRNA null mutants, which are viable, Lsm10 and Lsm11 mutants do not survive to adulthood. Because we cannot detect differences in the histone mRNA phenotype between Lsm10 or Lsm11 and U7 mutants, we propose that the different terminal developmental phenotypes result from the participation of Lsm10 and Lsm11 in an essential function that is distinct from histone pre-mRNA processing and that is independent of U7 snRNA.

Formation of the 3' end of histone mRNA: getting closer to the end

Nearly all eukaryotic mRNAs end with a poly(A) tail that is added to their 3' end by the ubiquitous cleavage/polyadenylation machinery. The only known exceptions to this rule are metazoan replication-dependent histone mRNAs, which end with a highly conserved stem-loop structure. This distinct 3' end is generated by specialized 3' end processing machinery that cleaves histone pre-mRNAs 4-5 nucleotides downstream of the stem-loop and consists of the U7 small nuclear RNP (snRNP) and number of protein factors. Recently, the U7 snRNP has been shown to contain a unique Sm core that differs from that of the spliceosomal snRNPs, and an essential heat labile processing factor has been identified as symplekin. In addition, cross-linking studies have pinpointed CPSF-73 as the endonuclease, which catalyzes the cleavage reaction. Thus, many of the critical components of the 3' end processing machinery are now identified. Strikingly, this machinery is not as unique as initially thought but contains at least two factors involved in cleavage/polyadenylation, suggesting that the two mechanisms have a common evolutionary origin. The greatest challenge that lies ahead is to determine how all these factors interact with each other to form a catalytically competent processing complex capable of cleaving histone pre-mRNAs.

Differences and similarities between Drosophila and mammalian 3' end processing of histone pre-mRNAs

We used nuclear extracts from Drosophila Kc cells to characterize 3' end processing of Drosophila histone pre-mRNAs. Drosophila SLBP plays a critical role in recruiting the U 7 snRNP to the pre-mRNA and is essential for processing all five Drosophila histone pre-mRNAs. The Drosophila processing machinery strongly prefers cleavage after a fourth nucleotide following the stem-loop and favors an adenosine over pyrimidines in this position. Increasing the distance between the stem-loop and the HDE does not result in a corresponding shift of the cleavage site, suggesting that in Drosophila processing the U 7 snRNP does not function as a molecular ruler. Instead, SLBP directs the cleavage site close to the stem-loop. The upstream cleavage product generated in Drosophila nuclear extracts contains a 3' OH, and the downstream cleavage product is degraded by a nuclease dependent on the U 7 snRNP, suggesting that the cleavage factor has been conserved between Drosophila and mammalian processing. A 2'O-methyl oligonucleotide complementary to the first 17 nt of the Drosophila U 7 snRNA was not able to deplete the U 7 snRNP from Drosophila nuclear extracts, suggesting that the 5' end of the Drosophila U 7 snRNA is inaccessible. This oligonucleotide selectively inhibited processing of only two Drosophila pre-mRNAs and had no effect on processing of the other three pre-mRNAs. Together, these studies demonstrate that although Drosophila and mammalian histone pre-mRNA processing share common features, there are also significant differences, likely reflecting divergence in the mechanism of 3' end processing between vertebrates and invertebrates.

Specificities of Caenorhabditis elegans and human hairpin binding proteins for the first nucleotide in the histone mRNA hairpin loop

The 3' ends of animal replication-dependent histone mRNAs are formed by endonucleolytic cleavage of the primary transcripts downstream of a highly conserved RNA hairpin. The hairpin-binding protein (HBP) binds to this RNA element and is involved in histone RNA 3' processing. A minimal RNA-binding domain (RBD) of approximately 73 amino acids that has no similarity with other known RNA-binding motifs was identified in human HBP [Wang Z-F et al., Genes & Dev, 1996, 10:3028-3040]. The primary sequence identity between human and Caenorhabditis elegans RBDs is 55% compared to 38% for the full-length proteins. We analyzed whether differences between C. elegans and human HBP and hairpins are reflected in the specificity of RNA binding. The C. elegans HBP and its RBD recognize only their cognate RNA hairpins, whereas the human HBP or RBD can bind both the mammalian and the C. elegans hairpins. This selectivity of C. elegans HBP is mostly mediated by the first nucleotide in the loop, which is C in C. elegans and U in all other metazoans. By converting amino acids in the human RBD to the corresponding C. elegans residues at places where the latter deviates from the consensus, we could identify two amino acid segments that contribute to selectivity for the first nucleotide of the hairpin loop.

Assembly of U7 small nuclear ribonucleoprotein particle and histone RNA 3' processing in Xenopus egg extracts

In animals, replication-dependent histone genes are expressed in dividing somatic cells during S phase to maintain chromatin condensation. Histone mRNA 3'-end formation is an essential regulatory step producing an mRNA with a hairpin structure at the 3'-end. This requires the interaction of the U7 small nuclear ribonucleoprotein particle (snRNP) with a purine-rich spacer element and of the hairpin-binding protein with the hairpin element, respectively, in the 3'-untranslated region of histone RNA. Here, we demonstrate that bona fide histone RNA 3' processing takes place in Xenopus egg extracts in a reaction dependent on the addition of synthetic U7 RNA that is assembled into a ribonucleoprotein particle by protein components available in the extract. In addition to reconstituted U7 snRNP, Xenopus hairpin-binding protein SLBP1 is necessary for efficient processing. Histone RNA 3' processing is not affected by addition of non-destructible cyclin B, which drives the egg extract into M phase, but SLBP1 is phosphorylated in this extract. SPH-1, the Xenopus homologue of human p80-coilin found in coiled bodies, is associated with U7 snRNPs. However, this does not depend on the U7 RNA being able to process histone RNA and also occurs with U1 snRNPs; therefore, association of SPH1 cannot be considered as a hallmark of a functional U7 snRNP.

Linker histone transitions during mammalian oogenesis and embryogenesis

A unique characteristic of the oocyte is that, although it is a differentiated cell, it can to give rise to a population of undifferentiated embryonic cells. This transition from a differentiated to a totipotential condition is thought to be mediated in part by changes in chromatin composition or configuration. In many non-mammalian organisms, oocytes contain unique subtypes of the linker histone H1, which are replaced in early embryos by the so-called somatic histone H1 subtypes. We review evidence that such histone H1 subtype switches also occur in mammals. Immunologically detectable somatic H1 is present in mitotically proliferating oogonia but gradually becomes undetectable after the oocytes enter meiosis. Immunoreactive somatic H1 remains undetectable throughout oogenesis and the early cell cycles after fertilization. Following activation of the embryonic genome, it is assembled onto chromatin. In contrast to the absence of immunoreactive protein, mRNAs encoding each of the five mammalian somatic H1 subtypes are present in growing oocytes and newly fertilized embryos, indicating that post-transcriptional mechanisms regulate expression of these genes. This maternal mRNA is degraded at the late 2-cell stage, and embryonically encoded mRNAs accumulate after embryos reach the 4-cell stage. During the period when somatic H1 is not detectable, oocytes and embryos contain mRNA encoding a sixth subtype, histone H1(0) which accumulates in differentiated somatic cells, and the nuclei can be stained with an H1(0)-specific antibody. We propose that the linker histone composition of the oocyte lineage resembles that of other mammalian cells, namely, that the somatic H1 subtypes predominate in mitotically active oogonia, that histone H1(0) becomes prominent in differentiated oocytes, and that following fertilization and transcriptional activation of the embryonic somatic H1 genes, the somatic H1 subtypes are reassembled onto chromatin of the embryonic cells. Potential functions of these linker histone subtype switches are discussed, including stabilization by H1(0) of the differentiated state of the oocytes, protection of the oocyte chromatin from factors that remodel sperm chromatin after fertilization, and restoration by the incorporation of the somatic H1 subtypes of the totipotential state of embryonic nuclei.

Formation of the 3' end of histone mRNA

All metazoan messenger RNAs, with the exception of the replication-dependent histone mRNAs, terminate at the 3' end with a poly(A) tail. Replication-dependent histone mRNAs end instead in a conserved 26-nucleotide sequence that contains a 16-nucleotide stem-loop. Formation of the 3' end of histone mRNA occurs by endonucleolytic cleavage of pre-mRNA releasing the mature mRNA from the chromatin template. Cleavage requires several trans-acting factors, including a protein, the stem-loop binding protein (SLBP), which binds the 26-nucleotide sequence; and a small nuclear RNP, U7 snRNP. There are probably additional factors also required for cleavage. One of the functions of the SLBP is to stabilize binding of the U7 snRNP to the histone pre-mRNA. In the nucleus, both U7 snRNP and SLBP are present in coiled bodies, structures that are associated with histone genes and may play a direct role in histone pre-mRNA processing in vivo. One of the major regulatory events in the cell cycle is regulation of histone pre-mRNA processing, which is at least partially mediated by cell-cycle regulation of the levels of the SLBP protein.

The stem-loop binding protein (SLBP1) is present in coiled bodies of the Xenopus germinal vesicle

The stem-loop binding protein (SLBP1) binds the 3' stem-loop of histone pre-mRNA and is required for efficient processing of histone transcripts in the nucleus. We examined the localization of SLBP1 in the germinal vesicle of Xenopus laevis oocytes. In spread preparations of germinal vesicle contents, an anti-SLBP1 antibody stained coiled bodies and specific chromosomal loci, including terminal granules, axial granules, and some loops. After injection of myc-tagged SLBP1 transcripts into the oocyte cytoplasm, newly translated myc-SLBP1 protein was detectable in coiled bodies within 4 h and in terminal and axial granules by 8 h. To identify the region(s) of SLBP1 necessary for subnuclear localization, we subcloned various parts of the SLBP1 cDNA and injected transcripts of these into the cytoplasm of oocytes. We determined that 113 amino acids at the carboxy terminus of SLBP1 are sufficient for coiled body localization and that disruption of a previously defined RNA-binding domain did not alter this localization. Coiled bodies also contain the U7 small nuclear ribonucleoprotein particle (snRNP), which participates in cleavage of the 3' end of histone pre-mRNA. The colocalization of SLBP1 and the U7 snRNP in the coiled body suggests coordinated control of their functions, perhaps through a larger histone-processing particle. Some coiled bodies are attached to the lampbrush chromosomes at the histone gene loci, consistent with the view that coiled bodies in the oocyte recruit histone-processing factors to the sites of histone pre-mRNA transcription. The non-histone chromosomal sites at which SLBP1 is found include the genes coding for 5 S rRNA, U1 snRNA, and U2 snRNA, suggesting a wider role for SLBP1 in the biosynthesis of small non-spliced RNAs.

Poly A-containing histone H4 mRNA variant (H4-v. 1): isolation and sequence determination from bovine adrenal medulla

A histone H4 cDNA variant (H4-v.1) was cloned from a bovine adrenal medullary phage library using PCR as a method of detection. The isolated clones contained a short 5' untranslated region (UTR) followed by the histone H4 coding region and a long atypical 3'UTR. The 3'UTR comprised the palindromic and purine-rich sequences typical of cell-cycle dependent histone mRNAs, and a 1.1 kb extension downstream of the palindromic sequence ending with a poly(A) track typical of cell-cycle independent histone mRNAs. Northern blot and RT-PCR analyses indicate that the transcript is fully expressed in bovine adrenal medulla. Thus, bovine histone H4-v.1 mRNA represents the first example of a histone H4 transcript that contains both 3'UTR characteristics of cell-cycle dependent and cell-cycle independent histone mRNAs.

Translational control of cellular and viral mRNAs

We are becoming increasingly aware of the role that translational control plays in regulating gene expression in plants. There are now many examples in which specific mechanisms have evolved at the translational level that directly impact the amount of protein produced from an mRNA. All regions of an mRNA, i.e., the 5' leader, the coding region, and the 3'-untranslated region, have the potential to influence translation. The 5'-terminal cap structure and the poly(A) tail at the 3' terminus serve as additional elements controlling translation. Many viral mRNAs have evolved alternatives to the cap and poly(A) tail that are functionally equivalent. Nevertheless, for both cellular and viral mRNAs, a co-dependent interaction between the terminal controlling elements appears to be the universal basis for efficient translation.

Characterization of a new class of transcribed repetitive DNA sequence which also exists as a hybrid with HP1 mRNA; potential for site-specific recombination in Drosophila melanogaster

A new class of dispersed repetitive DNA designated as vivi-sequence (VS) has been identified in Drosophila. It is relatively AT rich and is transcribed. The VS transcription is developmentally regulated and generates multiple transcripts. A hybrid transcript, designated fl-cDNA, has been identified in which a small segment of the VS is fused to the 5' end of an unrelated structural gene transcript coding for the heterochromatin protein HP1. The VS has recombination signal sequences (RSS) characteristic of vertebrate immunoglobulin genes. Such sequences are also present in the HP1 DNA. In both cases the recombination signal sequences are found close to the junction between HP1 and the VS in fl-cDNA. There is additional sequence identity both 5' and 3' of the junction between HP1 and the VS in fl-cDNA. We propose that (a) the HP1-VS composite transcript represented by the fl-cDNA may be the product of recombination between the two sequences, (b) that the process is mediated by the RSS and/or the DNA downstream of the junction between HP1 and the VS and (c) that the recombination event may lead to the inactivation of the HP1 gene in a cell and tissue specific manner.

Brain non-adenylated mRNAs

Most eukaryotic messenger RNA (mRNA) species contain a 3'-poly(A) tract. The histone mRNAs are a notable exception although a subclass of histone-encoding mRNAs is polyadenylated. A class of mRNAs lacking a poly(A) tail would be expected to be less stable than poly(A)+ mRNAs and might, like the histones, have a half-life that varied in response to changes in the intracellular milieu. Brain mRNA exhibits an unusually high degree of sequence complexity; studies published ten years ago suggested that a large component of this complexity might be present in a poly(A)- mRNA population that was expressed postnatally. The question of the existence of a complex class of poly(A)- brain mRNAs is particularly tantalizing in light of the heterogeneity of brain cells and the possibility that the stability of these poly(A)- mRNAs might vary with changes in synaptic function, changing hormonal stimulation or with other modulations of neuronal function. The mRNA complexity analyses, although intriguing, did not prove the existence of the complex class of poly(A)- brain mRNAs. The observed mRNA complexity could have resulted from a variety of artifacts, discussed in more detail below. Several attempts have been made to clone members of this class of mRNA. This search for specific poly(A)- brain mRNAs has met with only limited success. Changes in mRNA polyadenylation state do occur in brain in response to specific physiologic stimuli; however, both the role of polyadenylation and de-adenylation in specific neuronal activities and the existence and significance of poly(A)- mRNAs in brain remain unclear.

Molecular modeling to predict the structural and biological effects of mutations in a highly conserved histone mRNA loop sequence

The 3'-end of histone mRNAs contains a highly conserved sequence motif which is believed to form a 6 base pair stem and a 4 base loop. These sequences are involved in both the efficiency of 3'-end formation and stability of the mature histone mRNA. We have modeled four stem basepairs and the loop portion of this structure using the wildtype sequences and several mutant sequences. A structure for the wildtype stem-loop is proposed that is based on energy minimization using a representative wildtype sequence and comparison with structures obtained using naturally occurring mutations which do not alter loop function. A wildtype structure is proposed in which the top basepair of the stem is broken, forming a six base loop. Mutant sequences with altered bases in the loop and in the stem were also modeled. The effect of these mutations on the proposed wildtype structure is discussed and possible biological consequences considered.

Nucleotide sequences of Caenorhabditis elegans core histone genes. Genes for different histone classes share common flanking sequence elements

We have determined the nucleotide sequence of core histone genes and flanking regions from two of approximately 11 different genomic histone clusters of the nematode Caenorhabditis elegans. Four histone genes from one cluster (H3, H4, H2B, H2A) and two histone genes from another (H4 and H2A) were analyzed. The predicted amino acid sequences of the two H4 and H2A proteins from the two clusters are identical, whereas the nucleotide sequences of the genes have diverged 9% (H2A) and 12% (H4). Flanking sequences, which are mostly not similar, were compared to identify putative regulatory elements. A conserved sequence of 34 base-pairs is present 19 to 42 nucleotides 3' of the termination codon of all the genes. Within the conserved sequence is a 16-base dyad sequence homologous to the one typically found at the 3' end of histone genes from higher eukaryotes. The C. elegans core histone genes are organized as divergently transcribed pairs of H3-H4 and H2A-H2B and contain 5' conserved sequence elements in the shared spacer regions. One of the sequence elements, 5' CTCCNCCTNCCCACCNCANA 3', is located immediately upstream from the canonical TATA homology of each gene. Another sequence element, 5' CTGCGGGGACACATNT 3', is present in the spacer of each heterotypic pair. These two 5' conserved sequences are not present in the promoter region of histone genes from other organisms, where 5' conserved sequences are usually different for each histone class. They are also not found in non-histone genes of C. elegans. These putative regulatory sequences of C. elegans core histone genes are similar to the regulatory elements of both higher and lower eukaryotes. The coding regions of the genes and the 3' regulatory sequences are similar to those of higher eukaryotes, whereas the presence of common 5' sequence elements upstream from genes of different histone classes is similar to histone promoter elements in yeast.

Functional analysis of the sea urchin U7 small nuclear RNA

U7 small nuclear RNA (snRNA) is an essential component of the RNA-processing machinery which generates the 3' end of mature histone mRNA in the sea urchin. The U7 small nuclear ribonucleoprotein particle (snRNP) is classified as a member of the Sm-type U snRNP family by virtue of its recognition by both anti-trimethylguanosine and anti-Sm antibodies. We analyzed the function-structure relationship of the U7 snRNP by mutagenesis experiments. These suggested that the U7 snRNP of the sea urchin is composed of three important domains. The first domain encompasses the 5'-terminal sequences, up to about nucleotides 7, which are accessible to micrococcal nuclease, while the remainder of the RNA is highly protected and hence presumably bound by proteins. This region contains the sequence complementarities between the U7 snRNA and the histone pre-mRNA which have previously been shown to be required for 3' processing (F. Schaufele, G. M. Gilmartin, W. Bannwarth, and M. L. Birnstiel, Nature [London] 323:777-781, 1986). Nucleotides 9 to 20 constitute a second domain which includes sequences for Sm protein binding. The complementarities between the U7 snRNA sequences in this region and the terminal palindrome of the histone mRNA appear to be fortuitous and play only a secondary, if any, role in 3' processing. The third domain is composed of the terminal palindrome of U7 snRNA, the secondary structure of which must be maintained for the U7 snRNP to function, but its sequence can be drastically altered without any observable effect on snRNP assembly or 3' processing.