An electron microscope heteroduplex study of the ribosomal DNAs of Xenopus laevis and Xenopus mulleri

Characterization of the filarial genome

Filarial parasites are just beginning to be studied at the genetic level. The potential of recombinant DNA technology for identifying parasite genes that are important in the pathogenesis of filarial disease or for the survival of the parasite is enormous. Work in several laboratories has already identified genes which encode ribosomal RNAs, as well as highly repeated DNA sequences that can be used as diagnostic probes. In addition, new methods to separate chromosomes will allow the physical mapping of parasite genes without the requirement for classical genetic analysis, which would be difficult in filariids.

A U3 small nuclear ribonucleoprotein-requiring processing event in the 5' external transcribed spacer of Xenopus precursor rRNA

A processing site has been identified within the 5' external transcribed spacer (ETS) of Xenopus laevis and X. borealis pre-RNAs, and this in vivo processing can be reproduced in vitro. It involves a stable and specific association of the pre-rRNA with factors in the cell extract, including at least four RNA-contacting polypeptides, yielding a distinct complex that sediments at 20S. Processing also requires the U3 small nuclear RNA. This processing, at residue +105 of the 713-nucleotide X. laevis 5' ETS, is highly reminiscent of the initial processing cleavage of mouse pre-rRNA within its 3.5-kb 5' ETS, previously thought to be mammal specific. The frog and mouse processing signals share a short essential sequence motif, and mouse factors can faithfully process the frog pre-rRNA. This conservation suggests that this 5' ETS processing site serves an evolutionarily selective function.

A U3 small nuclear ribonucleoprotein-requiring processing event in the 5' external transcribed spacer of Xenopus precursor rRNA

A processing site has been identified within the 5' external transcribed spacer (ETS) of Xenopus laevis and X. borealis pre-RNAs, and this in vivo processing can be reproduced in vitro. It involves a stable and specific association of the pre-rRNA with factors in the cell extract, including at least four RNA-contacting polypeptides, yielding a distinct complex that sediments at 20S. Processing also requires the U3 small nuclear RNA. This processing, at residue +105 of the 713-nucleotide X. laevis 5' ETS, is highly reminiscent of the initial processing cleavage of mouse pre-rRNA within its 3.5-kb 5' ETS, previously thought to be mammal specific. The frog and mouse processing signals share a short essential sequence motif, and mouse factors can faithfully process the frog pre-rRNA. This conservation suggests that this 5' ETS processing site serves an evolutionarily selective function.

The origin of the rRNA precursor from Xenopus borealis, analysed in vivo and in vitro

We have determined the origin of the major transcript of Xenopus borealis rDNA by the use of an SI nuclease protection assay. The DNA surrounding the origin of this transcript was sequenced, and the region upstream of the origin was shown to have strong sequence homology with that region from X.laevis rDNA. We have also demonstrated faithful transcription from this origin using cloned X.borealis rDNA in an extract derived from X. laevis culture cells. This in vitro transcription was insensitive to 100 micrograms/ml alpha-amanatin, suggesting that it was mediated by RNA polymerase 1.

DNaseI-hypersensitive sites at promoter-like sequences in the spacer of Xenopus laevis and Xenopus borealis ribosomal DNA

We have detected a DNAseI hypersensitive site in the ribosomal DNA spacer of Xenopus laevis and Xenopus borealis. The site is present in blood and embryonic nuclei of each species. In interspecies hybrids, however, the site is absent in unexpressed borealis rDNA, but is present normally in expressed laevis rDNA. Hypersensitive sites are located well upstream (over lkb) of the pre-ribosomal RNA promoter. Sequencing of the hypersensitive region in borealis rDNA, however, shows extensive homology with the promoter sequence, and with the hypersensitive region in X. laevis. Of two promoter-like duplications in each spacer, only the most upstream copy is associated with hypersensitivity to DNAaseI. Unlike DNAaseI, Endo R. MspI digests the rDNA of laevis blood nuclei at a domain extending downstream from the hypersensitive site to near the 40S promoter. Since the organisation of conserved sequence elements within this "proximal domain" is similar in three Xenopus species whose spacers have otherwise evolved rapidly, we conclude that this domain plays an important role in rDNA function.

Tandemly repeated DNA sequences from Xenopus laevis. II. Dispersed clusters of a 388 base-pair repeating unit

A repetitive DNA sequence family from Xenopus laevis that has an unusual genomic organization has been identified. It has been shown by blot-hybridization that this sequence occurs in clusters containing variable numbers of the 388 base-pair repeating unit. There are approximately 500 such clusters in the genome, and each cluster has common flanking sequences. The number of tandem 388 base-pair repeats per cluster ranges from one to at least 15, with a mean of seven. Homologous sequences were found in two related species, Xenopus borealis and Xenopus mulleri, where the size of the repeating units and their genomic arrangement are very similar to those in X. laevis. The complete nucleotide sequence of a cloned representative 388 base-pair repeating unit showed no short internal repeats and no long reading frames. By blot-hybridization, no evidence of transcripts of this sequence was found in total RNA from X. laevis liver, embryos or oocytes.

Multiple heterogeneities in the transcribed spacers of ribosomal DNA from Xenopus laevis

Ribosomal DNA (rDNA) from Xenopus laevis contains several heterogeneities in all three transcribed spacers, as revealed by analysis of cloned and uncloned amplified rDNA from oocytes and cloned chromosomal rDNA from erythrocytes. Heterogeneities include single base changes and length variants of one to several nucleotides. Sites of variation are widely but non-uniformly distributed, some occurring only a short distance outside the boundaries of the rRNA coding regions. No two transcription units that we have yet examined are identical throughout their transcribed spacer regions.

Heterology of mitochondrial DNA from mammals detected by electron microscopic heteroduplex analyses

Heteroduplex analysis of mitochondrial DNA (mtDNA) from evolutionary closely related mammals (rat vs. mouse, man vs. monkey) are analyzed and compared to heteroduplex analysis of mt-DNA from more distantly related mammals (rat vs. man, rat vs. monkey, mouse vs. man, mouse vs. monkey and man vs. cow). Each analysis is transformed into a heteroduplex map and all maps are aligned to restriction enzyme maps and to genetic maps and where possible compared with the known sequence. We show that early evolutionary changes are seen mainly in URF2, URFA6L, URF6 and the D-loop region. The regions of rRNA, URF1, COI and COIII are generally very conserved regions but areas with some evolutionary activity can be localized. Heteroduplex analysis between distantly related species show much more heterology than do closely related species and the heteroduplex maps between all the distantly related species show a common pattern of heterology. Comparisons between the DNA sequence of mtDNA from man, cow and mouse and the equivalent heteroduplex maps show that base pair homologies higher than 73% are displayed as homologous regions. In the heteroduplex analysis of mtDNA's from more closely related species very few heterologies are displayed at 50% formamide but an increase in formamide concentration to 65-70% demonstrate also in these instances general heterologous regions.

Accurate transcription of cloned Xenopus rRNA genes by RNA polymerase I: demonstration by S1 nuclease mapping

We have demonstrated faithful transcriptional initiation of cloned Xenopus rRNA genes upon injection into Xenopus oocytes. This observation has been made possible by the use of an S1 nuclease assay that is both sensitive and quantitative. In order to detect rRNA synthesis from the injected template above the large background of rRNA endogenously present in oocytes, the divergence of ribosomal DNA sequences between two Xenopus species was utilized. Cloned X. laevis ribosomal DNA was injected into the nuclei of X. borealis oocytes. Total oocyte RNA was then isolated and hybridized to a radioactive DNA probe that over laps the 5' end of X. laevis rRNA; endogenous rRNA of the X. borealis oocytes does not hybridize to the probe. RNA/DNA hybrids were treated with S1 nuclease and protected fragments were sized by polyacrylamide gel electrophoresis. RNA made from the injected rDNA protects the same region of probe as does authentic X. laevis precursor rRNA. Thus, transcription appears to initiate on the cloned, microinjected X. laevis rDNA at the same site as is used in vivo. This synthesis is not impaired by coinjection of an amount of alpha-amanitin sufficient to inhibit RNA polymerase II and III; therefore the reaction is mediated by RNA polymerase I. The amount of transcription may be reproducibly quantitated and we have varied a number of parameters in order to maximize transcriptional expression of the injected rDNA. Eight independently isolated X. laevis rDNA clones as well as several subcloned initiation regions of these genes are all accurately transcribed at approximately equal efficiency. This assay should facilitate analysis of several aspects of rRNA transcription, including deleniation of the Xenopus RNA polymerase I promoter location.

DNAase I sensitivity and methylation of active versus inactive rRNA genes in xenopus species hybrids

We studied the chromatin structure and methylation of ribosomal RNA genes (rDNA) in hybrids between Xenopus laevis and Xenopus borealis. S1-nuclease protection experiments showed that 97%-98% of the rRNA precursor in hybrid tadpoles was of the X. laevis type. Preferential expression of the laevis rDNA was correlated with its hypersensitivity to DNAase I compared to borealis rDNA. Borealis and laevis rDNAs gave equivalent methylation patterns, however. The results show that hypomethylated sites in the nontranscribed spacer are not sufficient to ensure DNAase I hypersensitivity or transcription of the borealis rDNA. Also, heavy methylation of the transcribed region of laevis rDNA is compatible with its hypersensitivity to DNAase I. The absence of coupling between hypomethylation and DNAase I sensitivity argues against the view that the methylation pattern directly triggers the active chromatin structure, though it does not exclude a less intimate relationship between transcription and DNA hypomethylation. Examination of borealis sperm rDNA showed that hypomethylated sites were present at the same spacer locations as in somatic cells. This contrasts with X. laevis, where hypomethylated sites are detectable in the spacer of somatic rDNA, but not in sperm. Thus the loss of spacer methylation that is seen in early development of X. laevis does not occur in X. borealis.

The organization of macronuclear rDNA molecules of four hypotrichous ciliated protozoans

We have compared the structure of macronuclear DNA molecules that contain rRNA genes of four hypotricous ciliates, Stylonychia pustulata, Euplotes aediculatus, Oxytricha fallax and Oxytricha nova. The macronuclear rDNA, like all macronuclear DNA in hypotrichs, exists as achromosomal molecules of approximately single-gene size. The rDNA molecules have been cloned intact as recombinant plasmids and analyzed by restriction mapping and Southern hybridization. The sites of restriction enzymes BamHI, EcoRI, HindIII, PstI, PvuII and XhoI have similar but not identical patterns in Stylonychia and the two Oxytricha rDNAs. The restriction pattern of Euplotes rDNA is unlike those of the other three, with only one site of seventeen in the same position. Despite this divergence in nucleotide sequence, the overall structure of the rDNA molecules in the four hypotrichs is constant. The size of all the rDNA molecules is the same, 7.49 kb. Also, the positions of the regions coding for 19S and 25S rRNA are alike. The 25S coding region is at the 5' end of the DNA template strand (3' end of the RNA transcript), within 500 base pairs of the terminus of the DNA molecule. The 19S coding region is adjacent to the 25S region with less than 500 base pairs of spacer lying between the two genes. The largest non-coding sequence is at the 3' end of the DNA molecule adjoining the 19S RNA gene. The 3' non-coding regions show greater sequence divergence among the different rDNAs than do the coding regions. The similarity in size and organization of these molecules and the variability in the restriction patterns suggest that the gene structure is under tighter evolutionary constraint than is the primary nucleotide sequence.

Colobus type C virus: molecular cloning of unintegrated viral DNA and characterization of the endogenous viral genomes of Colobus

The unintegrated viral DNA intermediates of colobus type C virus (CPC-1) were isolated from infected human cells that were permissive for viral growth. There were two major species of DNA, linear molecules with two copies of the long terminal repeat and relaxed circles containing only a single long terminal repeat. In addition, there was a minor species (approximately 10%) composed of relaxed circles with two copies of the long terminal repeat. A restriction endonuclease map of the unintegrated DNA was constructed. The three EcoRI fragments of circular CPC-1 DNA were cloned in the EcoRI site of lambda gtWES . lambda B and then subcloned in the EcoRI site of pBR322. Using these subgenomic fragments as probes, we have characterized the endogenous viral sequences found in colobus cellular DNA. They are not organized in tandem arrays, as is the case in some other gene families. The majority of sequences detected in cellular DNA have the same map as the CPC-1 unintegrated DNA at 17 of 18 restriction endonuclease sites. There are, however, other sequences that are present in multiple copies and do not correspond to the CPC-1 map. They do not contain CPC-1 sequences either in an altered form or fused to common nonviral sequences. Instead, they appear to be derived from a distinct family of sequences that is substantially diverged from the CPC-1 family. This second family of sequences, CPC-2, is also different from the sequences related to baboon endogenous type C virus that forms a third family of virus-related sequences in the colobus genome.

Varicella zoster virus DNA exists as two isomers

Fragments of varicella zoster virus DNA produced by EcoRI endonuclease cleavage were cloned in vector pACYC 184 and those produced by HindIII cleavage were cloned in pBR322. Restriction enzyme cleavage maps established by double digestion and blot hybridization showed that varicella zoster virus DNA has a Mr of 80 +/- 3 x 10(6) and exists as a population of two isomers.

Nontranscribed spacers in Drosophila ribosomal DNA

Ribosomal DNA nontranscribed spacers in Drosophila virilis DNA have been examined in some detail by restriction site analysis of cloned segments of rDNA, nucleic acid hybridizations involving unfractionated rDNA, and base composition estimates. The overall G + C content of the spacer is 27--28%; this compares with 39% for rDNA as a whole, 40% for main band DNA, and 26% for the D. virilis satellites. Much of the spacer is comprised of 0.25 kb repeats revealed by digestion with Msp I, Fnu DII or Rsd I, which terminate very near the beginning of the template for the ribosomal RNA precursor. The spacers are heterogeneous in length among rDNA repeats, and this is largely accounted for by variation among rDNA units in the number of 0.25 kb elements per spacer. Despite its high A + T content and the repetitive nature of much of the spacer, and the proximity of rDNA and heterochromatin in Drosophila, pyrimidine tract analysis gave no indication of relatedness between the spacer and satellite DNA sequences. Species of Drosophila closely related to D. virilis have rDNA spacers that are homologous with those in D. virilis to the extent that hybridization of a cloned spacer segment of D. virilis rDNA to various DNA is comparable with hybridization to homologous DNA, and distributions of restriction enzyme cleavage sites are very similar (but not identical) among spacers of the various species. There is spacer length heterogeneity in the rDNA of all species, and each species has a unique major rDNA spacer length. Judging from Southern blot hybridization, D. hydei rDNA spacers have 20--30% sequence homology with D. virilis rDNA spacers, and a repetitive component is similarly sensitive to Msp I and Fnu DII digestion. D. melanogaster rDNA spacers have little or no homology with counterparts in D. virilis rDNA, despite a similar content of 0.25 kb repetitive elements. In contrast, sequences in rDNA that encode 18S and 28S ribosomal RNA have been highly conserved during the divergence of Drosophila species; this is inferred from interspecific hybridizations involving ribosomal RNA and a comparison of distributions of restriction enzyme cleavage sites in rDNA.

Comparison of the nuclear ribosomal units of five Chlamydomonas species

The organization of the nuclear ribosomal units of five different species of Chlamydomonas has been examined by hybridization of their nuclear DNA fragments produced by several restriction endonucleases with a radioactively labelled probe consisting of the two cloned BamHI ribosomal fragments of C. reinhardii. The results indicate that a) the ribosomal units of these five species are structurally related, b) changes in the non transcribed spacer occur in C. eugametos and in C. globosa, c) the rDNA unit of C. intermedia contains either an enlarged internal transcribed spacer or a ribosomal intervening sequence, d) the rDNA units of C. reinhardii and C. callosa are indistinguishable.

Organization of ribosomal genes in Paramecium tetraurelia

The macronuclear ribosomal DNA (rDNA) of the ciliated protozoan Paramecium tetraurelia (stock 51) was analyzed by digestion with restriction endonucleases. The fragments which contained ribosomal RNA (rRNA) coding sequences and spacer sequences were identified. The spacer sequences exhibited some heterogeneity in size. The genes coding for 5.8S RNA, but not for 5S RNA, are linked to the 17S and 25S rRNA genes. Complementary RNA, synthesized from rDNA of stock 51, was hybridized with restriction digests of whole cell DNA from six other allopatric stocks of this species. The restriction patterns of the rDNA from these seven stocks were, in general, very similar, and the sizes of the coding sequences were identical in all seven stocks. Only the restriction pattern of rDNA from stock 127 differed significantly from that of stock 51. The rDNA from stock 127 was isolated and characterized, and with the exception of the restriction pattern of its spacer, it resembled the rDNA from stock 51. It is concluded that the rDNA repeat in Paramecium, including the spacer, has, in general, been conserved during the course of evolution. It is suggested that in some species, even in the absence of genetic exchange among geographically separated populations, selection pressure may act to conserve spacers of tandemly repeated rDNA. The conservation may be related to the number of rDNA copies in the germinal nucleus.

Denaturation map of the ribosomal DNA of Lytechinus variegatus sperm

Electron microscopy of the partially heat denatured ribosomal DNA (rDNA) from sea urchin (Lytechinus variegatus) sperm has demonstrated that it consists of repeating units of 3.6 +/- 0.2 micron, corresponding to a mol.wt of 7.2 +/- 0.4 x 10(6). Based on differential denaturability, each repeat unit is divided into 2 regions. The larger region of 2.47 +/- 0.11 micron (mol.wt 4.9 +/- 0.22 x 10(6)) corresponds in length to the ribosomal precursor RNA of sea urchins and the smaller, GC-rich, subunit of 1.16 +/- 0.09 micrometer (mol.wt 2.3 +/- 0.18 x 10(6)) is presumed to contain non-transcribed spacer sequences.

Mapping of the Xenopus laevis 5.8S rDNA by restriction and DNA sequencing

The location of the 5.88 rDNA within the internal transcribed spacer has been found by restriction and sequence analysis. These analyses indicate the deletion of a dinucleotide from the known rRNA sequence. Regions to the 5' and 3' of the gene contain both uncommon sequences and palindromic structures which might provide potential control points. A secondary structure model is suggested for the 5.8S rRNA incorporating the flanking sequences.

Exploration of long and short repetitive sequence relationships in the sea urchin genome

Long and short repetitive sequences of sea urchin DNA were prepared by reassociation of 2000 nucleotide long fragments to Cot 4 and digestion with the single strand specific nuclease S1. The S1 resistant duplexes were separated into long repetitive and short repetitive fractions on Agarose A50. The extent of shared sequences was studied by reassociating a labeled preparation of short repetitive DNA with an excess of unlabeled long repetitive DNA. Less than 10% of the long repetitive DNA preparation was able to reassociate with the short repetitive DNA. Thus the long and short repetitive elements appear to be principally independent sequence classes in sea urchin DNA. Precisely reassociating repetitive DNA was prepared by four successive steps of reassociation and thermal chromatography on hydroxyapatite. This fraction (3% of the genome) was reassociated by itself or with a great excess of total sea urchin DNA. The thermal stability of the products was identical in both cases (Tm=81 degrees C), indicating that precisely repeated sequences do not have many imprecise copies in sea urchin DNA.

Lengths and patterns of transcriptional units in the amplified nucleoli of oocytes of Xenopus laevis

Transcriptionally active chromatin from peripheral amplified nucleoli of lampbrush-chromosome stage oocytes of Xenopus laevis was dispersed and spread in various solutions of low salt concentrations (including some with additions of detergents) and examined by electron microscopy. Nucleolar material from oocytes of animals with normal (2-nu) and mutant (1-nu) genetical constitution of nucleolus organizers was compared. Histograms showing the distributions of the length of matrix units, apparent spacer intercepts, and the total repeating units of the rDNA containing chromatin axes revealed a significant degree of heterogeity, with indications of subclasses and predominant repeat unit size classes of 3.3 amd 3.8 micron length. The correspondence of matrix unit length to the molecular weight of the first stable product of rDNA transcription was studied using gel electrophoresis of labelled pre-rRNA under non-denaturing and denaturing conditions. Evaluations of individual strands of nucleolar chromatin further demonstrated the existence of both (i) strands with obviously homogeneous repeating units of and (ii) strands with intra-axial heterogeneity of rDNA subunits. "Prelude complexes", i.e. groups of transcriptional complexes in apparent spacer intercepts, were not infrequently noted. The data are compared with the measurements of lengths of repeating units in fragments of rDNA obtained by digestion with EcoRI endonuclease as described by Morrow et al.(1974) and Wellauer et al. (1974,1976a, b). The results are discussed in relation to problems of variations in the modes of arrangement of the pre-rRNA genes, the state of packing of rDNA during transcription, and possible mechanisms of the amplification process.

Electron microscopy of late adenovirus type 2 mRNA hybridized to double-stranded viral DNA

R loops were generated with late adenovirus type 2 (Ad2) mRNA in double-stranded viral DNA, and visualized by electron microscopy. Unpaired DNA sequences in Ad2:Ad2+ND4 heteroduplex DNA served as a visual marker for the orientation of R loops with respect to the conventional DNA map. The most abundant classes of late Ad2 mRNA observed by this technique hybridized, in order of R-loop frequency, with midpoints near posit1ons 0.57, 0.88, 0.77, and 0.40 to 0.50 of the DNA map. The R loop at position 0.57, 0.88, 0.77, and 0.40 containing the hexon gene; the one at position 0.88 corresponded to a region containing the fiber gene. The relative frequencies of these two R loops paralleled those of the encoded gene products. The mRNA sizes, calculated from those of the respective R loops, were slightly larger than needed to code for these polypeptides. Using the R-loop technique, two locations at which adjacent mRNA's hybridized to different strands were accurately mapped at positions 0.61 and 0.91 of the DNA. The map positions of late Ad2 mRNA correlated well to published RNA and protein maps.

The extrachromosomal control of nonsense suppression in yeast: an analysis of the elimination of [psi+] in the presence of a nuclear gene PNM

When a [psi-] strain of yeast mutates to [psi+], the efficiency of suppression by certain ochre suppressors is increased. The [psi+] phenotype is inherited extrachromosomally. There is a nuclear gene, PNM, which, when mutant, causes loss of the [psi+] phenotype. PNM- is dominant to PNM+ and a heterozygous diploid gradually loses the ability over successive generations, to produce PNM+ [psi+] spores. This paper describes the kinetics of this elimination and the data obtained are discussed in relation to two models of the molecular nature of the [psi] genetic determinant--one considering the [psi] determinant as an autonomous nucleic acid, the other treating the possibility that the [psi] nucleic acid is that which codes for rRNA in the nuclear genome.

Characterization and mapping of mitochondrial ribosomal RNA and mitochondrial DNA in Drosophila melanogaster

We report the isolation and some properties of the mitochondrial ribosomal RNA (mt-rRNA) of Drosophila melanogaster, and a restriction map of the mtDNA of this organism which shows the position of the rRNA genes and of the A + T-rich region in the DNA. The mt-rRNAs are about 860 and 1500 nucleotides long and have the unusual composition of about 20% G+C. Some 25% of the mtDNA is very rich in A+T and is visualized as a contiguous early melting region in denaturation mapping. We mapped the three cleavage sites of the restriction endonuclease Hae III and the four sites of Hind III on mtDNA relative to each other and relative to the early melting region. The rRNA genes have been positioned on this map. The two rRNA genes are next to each other, separated by a gap of about 160 bases. We determined the polarity of the rRNA molecules: transcription proceeds in the direction small-to-large rRNA. Despite great divergence in nucleotide composition and sequence, the properties of mt-rRNA and the arrangement of mt-rRNA genes are very similar in Drosophila and vertebrate animals.

Repeating units of Xenopus laevis oocyte-type 5S DNA are heterogeneous in length

The restriction enzymes Hind III and Hae III cleave Xenopus laevis 5S DNA at one and three sites, respectively, in each repeating unit of approximately 700 base pairs. The cleavage sites for both enzymes have been located within the repeating unit by denaturation mapping of the restriction fragments. The Hind III products and one of the Hae III fragments are variable in length, indicating heterogeneity in the length of the repeating unit in 5S DNA. This length heterogeneity is confined to the major A + T-rich spacer region. Repeating units differ from each other by discrete quanta of approximately 15 base pairs. The A + T-rich spacer has been shown to consist largely of tandem subrepeats of just this size (Brownlee, Cartwright, and Brown, 1974). We suggest that the repeat-length heterogeneity is due to variable numbers of these subrepeats in the spacer regions of the major repeating units.

Characterization of yeast ribosomal DNA

High-molecular-weight yeast ribosomal DNA, purified by preparative Hg2+/Cs2SO4 gradient centrifugation, was characterized using RNA-DNA hybridization, CsC1 gradient analysis and denaturation-renaturation experiments. The following conclusions are drawn from the results of these studies. 1. 55--60% of the total yeast rDNA is made up of sequences coding for 42-S ribosomal precursor RNA. 2. The mean length of the 42-S precursor cistrons corresponds to a molecular weight of 6.2 X 10(6). 3. The overall G + C content fo yeast rDNA is 42%. On the hand, both the segments coding for the nonribosomal excess RNA of the 42-S precursor and those specifying 26-S and 17-S rRNA have a higher G + C content of 46.9--47.5%. 4. The 42-S precursor cistrons are closely bound up with sequences rather low in G + C content (34--35%), which account for 40--45% of the total yeast rDNA. In contrast to the multiple 42-S precurosr cistrons which show very little or no base sequence divergence these intervening (A + T)-rich sequences display a considerable heterogeneity in base sequence.