The genetics of dopa decarboxylase in Drosophila melanogaster. IV. The genetics and cytology of the 37B10-37D1 region

Regenerative growth is constrained by brain tumor to ensure proper patterning in Drosophila

Some animals respond to injury by inducing new growth to regenerate the lost structures. This regenerative growth must be carefully controlled and constrained to prevent aberrant growth and to allow correct organization of the regenerating tissue. However, the factors that restrict regenerative growth have not been identified. Using a genetic ablation system in the Drosophila wing imaginal disc, we have identified one mechanism that constrains regenerative growth, impairment of which also leads to erroneous patterning of the final appendage. Regenerating discs with reduced levels of the RNA-regulator Brain tumor (Brat) exhibit enhanced regeneration, but produce adult wings with disrupted margins that are missing extensive tracts of sensory bristles. In these mutants, aberrantly high expression of the pro-growth factor Myc and its downstream targets likely contributes to this loss of cell-fate specification. Thus, Brat constrains the expression of pro-regeneration genes and ensures that the regenerating tissue forms the proper final structure.

Short exposure to nitenpyram pesticide induces effects on reproduction, development and metabolic gene expression profiles in Drosophila melanogaster (Diptera: Drosophilidae)

Although the toxicity of neonicotinoid insecticides has been demonstrated in several studies, the information on metabolism, behavior, and health risk remains limited and has raised concerns about its potential toxicity. Thus, in this study we assessed the effects of nitenpyram using different sublethal concentrations (one-third and one-tenth of the acute LC₅₀ values) on various developmental and metabolic parameters from gene expression regulation in Drosophila melanogaster (model system used worldwide in ecotoxicological studies). As a result, nitenpyram sublethal concentrations prolonged the developmental time for both pupation and eclosion. Additionally, nitenpyram sublethal concentrations significantly decreased the lifespan, pupation rate, eclosion rate, and production of eggs of D. melanogaster. Moreover, the mRNA expression of genes relevant for development and metabolism was significantly elevated after exposure. Mixed function oxidase enzymes (Cyp12d1), (Cyp9f2), and (Cyp4ae1), hemocyte proliferation (RyR), and immune response (IM4) genes were upregulated, whereas lifespan (Atg7), male mating behavior (Ple), female fertility (Ddc), and lipid metabolism (Sxe2) genes were downregulated. These findings support a solid basis for further research to determine the hazardous effects of nitenpyram on health and the environment.

Molecular and biological functions of TRIM-NHL RNA-binding proteins

The TRIM-NHL family of proteins shares a conserved domain architecture and play crucial roles in stem cell biology, fertility, and development. This review synthesizes new insights that have revolutionized our understanding of the molecular and biological functions of TRIM-NHL proteins. Multiple TRIM-NHLs have been shown to bind specific RNA sequences and structures. X-ray crystal structures of TRIM-NHL proteins in complex with RNA ligands reveal versatile modes of RNA recognition by the NHL domain. Functional and genetic analyses show that TRIM-NHL RNA-binding proteins negatively regulate the protein expression from the target mRNAs that they bind. This repressive activity plays a crucial role in controlling stem cell fate in the developing brain and differentiating germline. To highlight these paradigms, we focus on several of the most-extensively studied TRIM-NHL proteins, specifically Drosophila and vertebrate TRIM71, among others. Brat is essential for development and regulates key target mRNAs to control differentiation of germline and neural stem cells. TRIM71 is also required for development and promotes stem cell proliferation while antagonizing differentiation. Moreover, TRIM71 can be utilized to help reprogram fibroblasts into induced pluripotent stem cells. Recently discovered mutations in TRIM71 cause the neurodevelopmental disease congenital hydrocephalus and emphasize the importance of its RNA-binding function in brain development. Further relevance of TRIM71 to disease pathogenesis comes from evidence linking it to several types of cancer, including liver and testicular cancer. Collectively, these advances demonstrate a primary role for TRIM-NHL proteins in the post-transcriptional regulation of gene expression in crucial biological processes. This article is categorized under: RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications Translation > Translation Regulation RNA Turnover and Surveillance > Regulation of RNA Stability.

Serotonin receptors expressed in Drosophila mushroom bodies differentially modulate larval locomotion

Drosophila melanogaster has been successfully used as a simple model to study the cellular and molecular mechanisms underlying behaviors, including the generation of motor programs. Thus, it has been shown that, as in vertebrates, CNS biogenic amines (BA) including serotonin (5HT) participate in motor control in Drosophila. Several evidence show that BA systems innervate an important association area in the insect brain previously associated to the planning and/or execution of motor programs, the Mushroom Bodies (MB). The main objective of this work is to evaluate the contribution of 5HT and its receptors expressed in MB to motor behavior in fly larva. Locomotion was evaluated using an automated tracking system, in Drosophila larvae (3^rd-instar) exposed to drugs that affect the serotonergic neuronal transmission: alpha-methyl-L-dopa, MDMA and fluoxetine. In addition, animals expressing mutations in the 5HT biosynthetic enzymes or in any of the previously identified receptors for this amine (5HT_1AR, 5HT_1BR, 5HT₂R and 5HT₇R) were evaluated in their locomotion. Finally, RNAi directed to the Drosophila 5HT receptor transcripts were expressed in MB and the effect of this manipulation on motor behavior was assessed. Data obtained in the mutants and in animals exposed to the serotonergic drugs, suggest that 5HT systems are important regulators of motor programs in fly larvae. Studies carried out in animals pan-neuronally expressing the RNAi for each of the serotonergic receptors, support this idea and further suggest that CNS 5HT pathways play a role in motor control. Moreover, animals expressing an RNAi for 5HT_1BR, 5HT₂R and 5HT₇R in MB show increased motor behavior, while no effect is observed when the RNAi for 5HT_1AR is expressed in this region. Thus, our data suggest that CNS 5HT systems are involved in motor control, and that 5HT receptors expressed in MB differentially modulate motor programs in fly larvae.

Dopa decarboxylase: a model gene-enzyme system for studying development, behavior, and systematics

Throughout its long evolutionary history, the Dopa decarboxylase gene (Ddc) has acquired a variety of functions in insects. The enzyme (DDC) catalyzes the production of the neural transmitters dopamine and serotonin. Not surprisingly, evidence of the enzyme's involvement in the behavior of insects is beginning to accumulate. In addition, DDC plays a role in wound healing, parasite defense, pigmentation, and cuticle hardening. A high degree of sequence conservation has allowed comparisons of the Ddc-coding regions from various insects, facilitating a number of recent studies on insect systematics. This review outlines the diverse functions of Ddc and illustrates how studies of this model system address many questions on insect neurobiology, developmental biology, and systematics.

Electron micrograph map of the Drosophila melanogaster polytene chromosome 3R divisions 81 through 90

The banding pattern of the proximal half of the polytene salivary gland 3R chromosome of Drosophila melanogaster was studied with thin section electron microscopy. Bands were identified according to Bridges' revised light microscopic map, which contains 330 single and 121 double bands within the regions 81 through 90. We found a total of 447 bands in this region. 97 Bridges' single bands were easily detected in almost all thin sections, while 177 faint bands could be seen only in some micrographs. 56 Bridges' single bands could not be found. 32 Bridges' doublets were made up of two separate bands each in thin sections. The other 89 Bridges' doublets seemed to be either single bands or remained obscure. A total of 20 small new bands, which were not drawn on Bridges' map, were detected.

Mutations in the beta-propeller domain of the Drosophila brain tumor (brat) protein induce neoplasm in the larval brain

Inactivation of both alleles of the fruit fly D. melanogaster brain tumor (brat) gene results in the production of a tumor-like neoplasm in the larval brain, and lethality in the larval third instar and pupal stages. We cloned the brat gene from a transposon-tagged allele and identified its gene product. brat encodes for an 1037 amino acid protein with an N-terminal B-boxl zinc finger followed by a B-box2 zinc finger, a coiled-coil domain, and a C-terminal beta-propeller domain with six blades. All these motifs are known to mediate protein-protein interactions. Sequence analysis of four brat alleles revealed that all of them are mutated at the beta-propeller domain. The clustering of mutations in this domain strongly suggests that it has a crucial role in the normal function of Brat, and defines a novel protein motif involved in tumor suppression activity. The brat gene is expressed in the embryonic central and peripheral nervous systems including the embryonic brain. In third instar larva brat expression was detected in the larval central nervous system including the brain and the ventral ganglion, in two glands - the ring gland and the salivary gland, and in parts of the foregut - the gastric caecae and the proventriculus. A second brat-like gene was found in D. melanogaster, and homologs were identified in the nematode, mouse, rat, and human. Accumulated data suggests that Brat may regulate proliferation and differentiation by secretion/transport-mediated processes.

derailed is required for muscle attachment site selection in Drosophila

During development, muscles must form and attach at highly stereotyped positions to allow for coordinated movements. In Drosophila, muscles grow towards and attach to specifically positioned cells within the epidermis. At the molecular level, very little is known about how muscles recognize these attachment sites. The derailed gene encodes a receptor tyrosine kinase family member that is essential for the pathfinding ability of expressing neurons. Here we show that the Drl RTK is also expressed by a small subset of developing embryonic muscles and neighboring epidermal cells during muscle attachment site selection. In drl mutants, these muscles often fail to attach at appropriate locations although their epidermal attachment cells appear unaffected. These results show that, similar to its role in neuronal pathway recognition, the Drl RTK participates in a mechanism required for muscle attachment site selection. The data suggest that both neurons and muscles use common mechanisms to recognize their paths or targets, and that Drl plays an analogous role in both developing systems.

Mutations in the Drosophila hook gene inhibit endocytosis of the boss transmembrane ligand into multivesicular bodies

Transmembrane ligands can be internalized across cell boundaries into receptor-expressing cells. In the developing Drosophila eye imaginal disc, the bride of sevenless transmembrane protein (boss) is expressed on the surface of R8 cells. After internalization into neighboring R7 cells, the boss protein accumulates in multivesicular bodies. In a search for genes that affect this cell-type-specific pattern of boss endocytosis, we found that mutations in the hook gene inhibit the accumulation of boss in multivesicular bodies of R7 cells. In addition, hook flies exhibit pleiotropic phenotypes including abnormal bristle morphology and eye degeneration. The wild-type-pattern of boss endocytosis was restored in hook mutants by a genomic rescue fragment containing the hook gene or by a hook cDNA expressed in R7 cells under control of a sevenless (sev) enhancer. The hook gene encodes a novel cytoplasmic protein of 679 amino acids with a central coiled-coil domain of some 200 amino acids. Truncated, epitope-tagged hook proteins coimmunoprecipitated the full-length protein, indicating dimerization mediated by the coiled-coil domain. The hook protein localizes to vesicular structures that are part of the endocytic compartment. The requirement of the hook protein in R7 cells for the accumulation of boss protein in multivesicular bodies, and the localization of the hook protein to endocytic vesicles indicate that the hook gene encodes a novel component of the endocytic compartment that plays an important role in the endocytosis of transmembrane ligands or their transport to multivesicular bodies.

Control of neuronal pathway selection by a Drosophila receptor protein-tyrosine kinase family member

During development, neurons are capable of selecting specific pathways that lead them to their appropriate target areas. A variety of molecular mechanisms are thought to be involved in pathway recognition, including cell adhesion, repulsion and chemotropism. However, apart from a few genes whose involvement has been shown genetically, the mechanisms underlying neuronal pathway selection are largely unknown. Here we report the isolation of the Drosophila derailed (drl) gene, which encodes a novel member of the receptor protein-tyrosine kinase family. Using a newly developed axon-targeted reporter gene we find that drl is expressed by a small subset of embryonic interneurons whose growth cones choose common pathways during development. In drl mutant embryos these neurons fail to make the correct pathway choices. Our results provide evidence for receptor protein-tyrosine kinase involvement in key aspects of neuronal pathway recognition.

Effects of Ddc cluster lethal alleles on ovary growth, attachment, and egg production in Drosophila

The Ddc cluster of genes on the left arm of chromosome 2 in Drosophila has been extensively characterized by Wright and coworkers (Wright, '87b). Many of the genes in the cluster are associated with the pigmentation and sclerotization of the cuticle, and at least 12 have been shown to play a role in female fertility. To characterize further the actions of genes in the cluster, we have investigated the effect on fertility of a total of five of the previously untested genes (l(2)37Be, Bb, Bg, Bd, and Cg), and three of the partially characterized genes (l(2)37Ce, Ddc, and amd). Each allele was crossed to Df(2L)TW130 or49h flies in order to make it hemizygous over the 8-12 band deficiency covering the Ddc region, 37B9-C1,2;37D1-2. Ovaries taken from larvae produced by this cross were transplanted into female larval hosts of y f mal genotype, that were then mated to v f mal males. The wild type allele of mal in implanted tissue allowed identification and study of surviving implants by staining for the presence of aldehyde oxidase. Of the 18 alleles available, amdH149, l(2)37Bb1, Bb9, Bb11, Bd6, Be1, Bd7, Be2, Be3, Ce4, and Cg1 did not allow enough growth to form transplantable ovaries; l(2)37Bg1, Bg2, and Cgts1 prevented development of transplanted ovaries in their hosts; l(2)37Ce5 allowed implanted ovaries to attach to oviducts and grow, but insufficiently for production of eggs; and DdcN27, amd29, and l(2)37Bd4 appeared not to restrict ovary development. Heteroallelic heterozygotes of Bd7 x Bd4 also produced fully fertile ovaries, but no other heteroallelic combinations did so.(ABSTRACT TRUNCATED AT 250 WORDS)

The Drosophila spitz gene encodes a putative EGF-like growth factor involved in dorsal-ventral axis formation and neurogenesis

We describe the molecular characterization of the Drosophila gene spitz (spi), which encodes a putative 26-kD, EGF-like transmembrane protein that is structurally similar to TGF-alpha. Temporal and spatial expression patterns of spi transcripts indicate that spi is expressed throughout the embryo. Examination of mutant embryos reveals that spi is involved in a number of unrelated developmental choices, for example, dorsal-ventral axis formation, glial migration, sensory organ determination, and muscle development. We propose that spi may act as a ligand for cell-specific receptors, possibly rhomboid and/or the Drosophila EGF receptor homolog.

A P element chimera containing captured genomic sequences was recovered at the vestigial locus in Drosophila following targeted transposition

A P element carrying the Dopa decarboxylase gene, P[Ddc], was targeted into vg21, a cryptic P element induced mutant allele of the vestigial (vg) locus. The resulting allele, vg28w, contained the expected P[Ddc] plus an additional 9.5 kb of DNA, captured from elsewhere on chromosome II. Reversion of the vg28w mutant allele demonstrated that the entire insert can excise but cannot reinsert at an appreciable frequency. We explain the targeted transposition as the repair of a double stranded gap, created by the excision of the P element at vg21, and suggest that the formation of chimeric elements may be an important component of P element dependent genomic instability.

Characterization of third chromosome dominant alpha-methyl dopa resistant mutants (Tcr) and their interactions with l(2)amd alpha-methyl dopa hypersensitive alleles in Drosophila melanogaster

In Drosophila melanogaster two alleles at the Third chromosome resistance locus (Tcr; 3-39-6) were isolated in a screen of EMS mutagenized third chromosomes for dominant resistance to dietary alpha-methyl dopa, alpha-MD, a structural analogue of DOPA. Both alleles of Tcr are recessive lethals exhibiting partial complementation. Almost half (48.3%) of the Tcr40/Tcr45 heterozygotes die as embryos but some survive past adult eclosion. Both the embryonic lethal phenotype and the adult phenotype suggest that Tcr is involved in cuticle synthesis. Tcr mutants suppress the lethality of partially complementing alleles at the alpha-MD hypersensitive locus, l(2)amd. The viability of Tcr40/Tcr45, however, is not increased by the presence of a l(2)amd allele. The possibility that the Tcr and l(2)amd mutations reveal a catecholamine metabolic pathway involved in cuticle structure is discussed.

Overlapping transcription units in Drosophila: sequence and structure of the Cs gene

The Cs gene lies between the functionally and evolutionarily related dopa decarboxylase (Ddc) and l(2)amd loci of Drosophila. The Cs and Ddc genes overlap at their 3' ends, implying that the transcription termination signals of these genes are polar, since each gene's primary transcript contains the complement of the other gene's transcription termination signals. The mature transcripts of the Cs and Ddc genes are complementary for a short distance and the primary transcripts may be complementary over thousands of base pairs. Despite intensive mutagenesis in this region, no mutations affecting the Cs transcript have been recovered although over 90 alleles of the two flanking genes (Ddc and l(2)amd) have been identified. Unlike the flanking Ddc and l(2)amd genes, the structure of the Cs gene and the temporal and tissue specificity of Cs expression are inconsistent with any structural or functional relatedness to the Ddc gene family. The internal structure of the Cs transcript is unlike that of most protein coding genes; it contains several open reading frames which are not situated favorably for efficient translation of the Cs message. This unusual internal structure may be the basis of the observed mutational silence of the Cs locus.

The alpha methyl dopa hypersensitive gene, 1(2)amd, and two adjacent genes in Drosophila melanogaster: physical location and direct effects of amd on catecholamine metabolism

The dopa decarboxylase gene (Ddc) is located in a very dense cluster of genes many of whose functions appear to be related to the physiological role of dopa decarboxylase (DDC) in catecholamine metabolism. In Drosophila melanogaster catecholamine metabolism is involved in the production of neurotransmitters and in the synthesis of cross-linking agents for cuticular sclerotization. In this report we consider three loci near Ddc that affect cuticle formation. The alpha methyl dopa hypersensitive gene, 1(2)amd, is definitively assigned to a transcriptional unit 2 kb distal to Ddc. The assignment of 1(2) 37 Bd and 1(2)37 Cc to coding regions in the immediate vicinity of amd and Ddc is examined. amd+ gene activity performs a vital function essential for the formation of insect cuticle and also determines the level of sensitivity to the DDC analogue inhibitor, alpha methyl dopa. We present data that provide direct evidence that the amd+ gene product is required for a step in the metabolism of dopa to one or more novel catecholamines involved in the colorless sclerotization of cuticle.

A test for the role of natural selection in the stabilization of transposable element copy number in a population of Drosophila melanogaster

The numbers of members of three families ofcopia-like elements were counted on twentyX, 2nd and 3rd chromosomes collected from a natural population ofDrosophila melanogaster. Theoretical predictions were computed for two models of copy number stabilization: (1) element frequencies are regulated by a simple genetic process such as copy number dependent transposition or excision, independent of chromosomal location; (2) elements are eliminated by natural selection against mutational effects of their insertion into the chromosome. Since insertions into theXcan be expected to suffer more selection than autosomal insertions, due to expression of mutant phenotypes in the hemizygous state, hypothesis 2, called the disproportional model, predicts that the proportion of elements on theXwill be smaller than the proportion of the genome contributed by theX, while hypothesis 1, called the equiproportional model, predicts that this proportionality will be unaffected. Two of the elements,297androo, showed no evidence for deficiency ofX-linked elements, but the data for a third element,412, were consistent with the prediction based on the selective model.

These results indicate that simple selection against mutational effects of insertions of transposable elements is not generally adequate to account for their distribution within populations. We argue that a mechanism such as recombination between elements at different chromosomal sites, leading to rearrangements with highly deleterious, dominant effects could play a role in stabilizing copy number. This process would lead to a higher abundance of elements in genomic regions with restricted crossing over. We present some data indicating such an effect, and discuss possible interpretations.

Sequence and expression of the Cc gene, a member of the dopa decarboxylase gene cluster of Drosophila: possible translational regulation

A transcript has been localized proximal to the dopa decarboxylase (Ddc) gene within a cluster of genes involved in cuticle formation and catecholamine metabolism in Drosophila. This gene, which has been identified as I(2)37Cc, maps 2.0kb from the 5' end of the Ddc gene and is transcribed in the same direction as Ddc. We describe a new deficiency which in conjunction with previous deficiencies localizes the I(2)37Cb and I(2)37Cc loci to the cytogenetic interval 5' to Ddc. We present the sequence of the Cc gene and corresponding cDNA. The Cc message contains several open reading frames 5' to the large open reading frame responsible for the lethal complementation group, suggesting that expression of Cc function may be regulated translationally. The Cc transcript is expressed in early embryos, late embryos, late third instar larvae and adults. We discuss the implications of these findings with respect to the gene organization in the region.

A kinetic analysis of Drosophila melanogaster dopa decarboxylase

The kinetic mechanism of dopa decarboxylase (3,4-dihydroxy-L-phenylalanine carboxy-lyase, EC 4.1.1.28) was investigated in Drosophila melanogaster. Based on initial velocity and product inhibition studies, an ordered reaction is proposed for dopa decarboxylase. This kinetic mechanism is interpreted in the context of measured enzyme activities and the catecholamine pools in Drosophila. The 1(2)amd gene is immediately adjacent to the gene coding for dopa decarboxylase (Ddc) and determines hypersensitivity to alpha-methyldopa in Drosophila. Dopa decarboxylase does not decarboxylate alpha-methyldopa and hence does not generate a toxic product capable of inhibiting 1(2)amd gene function. We propose that the 1(2)amd gene is involved with an unknown catecholamine pathway involving dopa but not dopamine.

Analysis of the transcription unit adjacent to the 3'-end of the dopa decarboxylase gene in Drosophila melanogaster

Using strand specific RNA probes, we have ascertained that the transcription unit immediately downstream of the dopa decarboxylase (DDC) gene in Drosophila, is oriented in the opposite direction to DDC. The 3'-termini of these two transcription units lie within 550 bp of one another, and the mature transcripts actually may overlap. We show here that the inclusion of part of the 3'-flanking transcript on a genomic DNA clone originally assumed to include just DDC, accounts for most of the hybridization seen in our previous Northern analysis of very early embryonic RNA. Thus, while DDC-specific transcripts are detectable in the early embryonic stages, they are much less prevalent than we have previously reported.

Developmental control of transduced dopa decarboxylase genes in D. melanogaster

Seventeen new euchromatic integration sites of the dopa-decarboxylase gene (Ddc) have been generated using p-mediated transduction. The developmental expression of the integrated genes was examined by monitoring the embryonic induction of dopa decarboxylase enzyme activity (DDC) and by monitoring the developmental pattern of DDC activity from late third instar to eclosion. The majority of inserts are regulated correctly within about 30% of controls. Several cases of multiple insertion events were recovered and these show correspondingly elevated levels of activity and are regulated normally. The pattern of expression of one insert (15C) falls outside the normal range. Multiple copies of transduced Ddc genes are used to test for effects of elevated gene dose on levels of expression. One insert on the X chromosome shows little or no dosage compensation. Possible reasons for the differences between the regulation of transduced genes in Drosophila and the regulation of transformed genes in mammalian systems are discussed.

Transcripts, genes and bands in 315,000 base-pairs of Drosophila DNA

We have mapped transcripts arising from 315,000 base-pairs of DNA from chromosome region 87D,E of Drosophila melanogaster. The DNA is represented in a series of overlapping recombinant phages; it constitutes about 14 bands in the polytene chromosome from 87D5,6 to 87E5,6 and contains the essential sequences for at least 12 complementation groups. We have defined 20 discrete polyadenylated RNA species transcribed from non-repetitive DNA in the region at various developmental stages. There is a generally good correlation between the position of transcription units, chromomeric units and complementation groups but with some significant exceptions. In particular, the two large bands in the region (E1,2 and E5,6) each contain several transcription units. We also find that a major part of a large band (E1,2) has no detectable transcripts and is apparently genetically silent.

Genetic interpretation of polytene chromosomes banding pattern

This mini-review covers new data regarding the problem of the functional organization of polytene chromosomes: The localization of RNA synthesis in the polytene chromosome puffs, diffuse bands and interbands; The relative stability of banding pattern and its functional value; The informational content of bands.

Genetics of the nervous system in Drosophila

The genetic analysis ofDrosophilaneurobiology has expanded into a considerable field. A host of genetic variants is now available for analyzing the development, structure, physiology, and chemistry of the fly's excitable cells and tissues and how they control the animal's behavior.

Correlation of hnRNP structure and nascent transcript cleavage

Using electron microscopy of spread chromatin, we have observed nonnucleolar transcription units from Drosophila melanogaster and Calliphora erythrocephala that display specific cleavage of nascent transcripts. We have quantitatively analyzed 20 of these relatively long transcription units. The primary RNP structure of homologous transcripts is nonrandom with respect to both RNA sequence and the cleavage event. In general, released RNA fragments have a smooth fibrillar RNP morphology (approximately 50 A wide) and retained segments have a thicker particulate morphology (approximately 250 A diameter). A characteristic secondary structure formation also accompanies cleavage--that is, RNP fibril loops form by association of noncontiguous transcript sequences that correspond to the terminal regions of the segment to be released. RNP particles form at the loop base sequences prior to their association and apparently coalesce upon loop formation. These loops, and thus the released segments, range in length from 1 and 25 kb on the examples we have analyzed. Cleavage of nascent hnRNA transcripts appears to be a fairly common event in these organisms and occurs within 0.3-3 min after transcription of the cleavage site.