Assembly of the mitochondrial membrane system. Processing of the apocytochrome b precursor RNAs in Saccharomyces cerevisiae D273-10B

Heterologous expression of mitochondrial nicotinamide adenine dinucleotide transporter (Ndt1) from Aspergillus fumigatus rescues impaired growth in Δndt1Δndt2 Saccharomyces cerevisiae strain

Our understanding of nicotinamide adenine dinucleotide mitochondrial transporter 1 (Ndt1A) in Aspergillus fumigatus remains poor. Thus, we investigated whether Ndt1A could alter fungi survival. To this end, we engineered the expression of an Ndt1A-encoding region in a Δndt1Δndt2 yeast strain. The resulting cloned Ndt1A protein promoted the mitochondrial uptake of nicotinamide adenine dinucleotide (NAD⁺), generating a large mitochondrial membrane potential. The NAD⁺ carrier utilized the electrochemical proton gradient to drive NAD⁺ entrance into mitochondria when the mitochondrial membrane potential was sustained by succinate. Its uptake has no impact on oxidative stress, and Ndt1A expression improved growth and survival of the Δndt1Δndt2 Saccharomyces cerevisiae strain.

A maturase-like coding sequence downstream of the OXI2 gene of yeast mitochondrial DNA is interrupted by two GC clusters and a putative end-of-messenger signal

By completing and correcting the sequence of a 1.8 kb DNA segment downstream of the oxi2 gene of Saccharomyces cerevisiae, a long, potentially coding sequence ("RF2") has been identified. The sequence is rather closely related to the RF1 open reading frame, downstream of the oxil gene, and, further, to the major family of intronic open reading frames. The RF2 open reading frame is not continuous, however, for it is interrupted by two GC clusters, both of which ultimately result in a -1 frameshift. Comparison with RF1 reveals a third insertion. This is centered on an oligo nucleotide, AATAATATTCTTA, which is found (sometimes in a slightly modified form) downstream of ten proven or suspected protein coding genes, including RF1 and RF2, and is known to terminate the apocytochrome b messenger RNA. It is suggested from the known distribution of this putative "end-of-messenger" signal, that it could play an essential part in controlling the expression of several minor proteins, both intronic and non-intronic. The possibility of the RF2 sequence being functional in spite of its interruptions is also discussed.

Pet127 governs a 5' -> 3'-exonuclease important in maturation of apocytochrome b mRNA in Saccharomyces cerevisiae

The details of mRNA maturation in Saccharomyces mitochondria are not well understood. All seven mRNAs are transcribed as part of multigenic units. The mRNAs are processed at a common 3'-dodecamer sequence, but the 5'-ends have seven different sequences. To investigate whether apocytochrome b (COB) mRNA is processed at the 5'-end from a longer precursor by an endonuclease or an exonuclease, a 64-nucleotide sequence, which is required for the protection of COB mRNA by the Cbp1 protein and is found at the 5'-end of the processed COB mRNA, was duplicated in tandem. The wild-type 64-nucleotide element functioned in either the upstream or downstream position when paired with a mutant element. In the tandem wild-type strain, the 5'-end of the mRNA was at the 5'-end of the upstream unit, demonstrating that the mRNA is processed by an exonuclease. Accumulation of precursor COB RNA in single and double element strains with a deletion of PET127 demonstrated that the encoded protein governs the 5'-exonuclease responsible for processing the precursor to the mature form.

CBT1 interacts genetically with CBP1 and the mitochondrially encoded cytochrome b gene and is required to stabilize the mature cytochrome b mRNA of Saccharomyces cerevisiae

Mutation of a CCG sequence in the 5'-untranslated region of the mitochondrially encoded cytochrome b mRNA in Saccharomyces cerevisiae results in destabilization of the message and respiratory deficiency of the mutant strain. This phenotype mimics that of a mutation in the nuclear CBP1 gene. Here it is shown that overexpression of the nuclear CBT1 gene, due to a transposon insertion in the 5'-untranslated region, rescues the respiratory defects resulting from mutating the CCG sequence to ACG. Overexpressing alleles of CBT1 are allelic to soc1, a previously isolated suppressor of cbp1ts-induced temperature sensitivity of respiratory growth. Quantitative primer extension analysis indicated that cbt1 null strains have defects in 5'-end processing of precursor cytochrome b mRNA to the mature form. Cbt1p is also required for stabilizing the mature cytochrome b mRNA after 5' processing.

Protein-dependent transition states for ribonucleoprotein assembly

Native folding and splicing by the Saccharomyces cerevisiae mitochondrial bI5 group I intron RNA is facilitated by both the S. cerevisiae CBP2 and Neurospora crassa CYT-18 protein cofactors. Both protein-bI5 RNA complexes splice at similar rates, suggesting that the RNA active site structure is similar in both ribonucleoproteins. In contrast, the two proteins assemble with the bI5 RNA by distinct mechanisms and bind opposing, but partially overlapping, sides of the group I intron catalytic core. Assembly with CBP2 is limited by a slow, unimolecular RNA folding step characterized by a negligible activation enthalpy. We show that assembly with CYT-18 shows four distinctive features. (1) CYT-18 binds stably to the bI5 RNA at the diffusion controlled limit, but assembly to a catalytically active RNA structure is still limited by RNA folding, as visualized directly using time-resolved footprinting. (2) This mechanism of rapid stable protein binding followed by subsequent assembly steps has a distinctive kinetic signature: the apparent ratio of k(off) to k(on), determined in a partitioning experiment, differs from the equilibrium K(d) by a large factor. (3) Assembly with CYT-18 is characterized by a large activation enthalpy, consistent with a rate limiting conformational rearrangement. (4) Because assembly from the kinetically trapped state is faster at elevated temperature, we can identify conditions where CYT-18 accelerates (catalyzes) bI5 RNA folding relative to assembly with CBP2.

The DExH box protein Suv3p is a component of a yeast mitochondrial 3'-to-5' exoribonuclease that suppresses group I intron toxicity

The yeast mitochondrial protein Suv3p is a putative NTP-dependent RNA helicase. Here we report that in cells lacking Suv3p, there is an approximately 50-fold increase in the excised form of the group I intron omega of the mitochondrial 31S rRNA gene. Surprisingly, little mature 21S rRNA accumulates in those cells; instead, unligated 21S rRNA exons appear. Intron overaccumulation could lead to spliced exon reopening via a reaction known to be catalyzed by group I introns in vitro. We also show that Suv3p is a functional component of a novel mitochondrial NTP-dependent 3'-to-5' exoribonuclease activity that can degrade group I intron RNAs. These findings account for group I intron overaccumulation in cells lacking Suv3p and define a novel function for putative RNA helicases in direct RNA degradation.

The mobile group I intron 3 alpha of the yeast mitochondrial COXI gene encodes a 35-kDa processed protein that is an endonuclease but not a maturase

Three mitochondrial mutants were characterized that block the splicing of aI3 alpha, a mobile group I intron of the COXI gene of yeast mtDNA. Mutant C1085 alters helical structures known to be important for splicing of group I introns. M44 and C1072 are point mutants in exon 3 that block correct splicing but allow some splicing at cryptic 5'-splice sites. M44 alters the P1 helix needed for 5'-splice site definition, while the mutation in C1072 is a new kind of mutation because it is located upstream of the exon sequence involved in the P1 helix. All three mutants accumulate novel proteins of 35 and 44 kDa (p35 and p44, respectively) detected both by labeling of mitochondrial translation products and by Western blotting. Partial protease digestions indicate that p44 and p35 are closely related, probably as precursor and processed protein. The level of the intron-encoded endonuclease activity, I-SceIII, is elevated approximately 10-fold in the mutants. Partial purification of I-SceIII from the mutants showed that most, if not all, of the activity is associated with p35. Finally, because aI3 alpha splices accurately in a petite mutant, we conclude that aI3 alpha splicing does not depend on a mtDNA-encoded maturase.

Formation of the 3' end of yeast mitochondrial mRNAs occurs by site-specific cleavage two bases downstream of a conserved dodecamer sequence

Mitochondrial mRNAs in yeast arise by processing of polygenic primary transcripts at a conserved dodecamer sequence (5'-AAUAAPyAUUCUU-3'). Previous results indicated that processing at dodecamer sites interrupted the sequence implying that it functioned primarily as a signal for 3' end formation of mRNAs. We have determined the precise cleavage site for RNAs processed at the dodecamer sequences associated with the oli1 gene and the omega intron of the 21S rRNA gene. In both cases cleavage occurred two bases downstream of the site. Hydrolysis left the PO4 group attached to the 3' terminus of the cleavage products. These results demonstrate for the first time that mature mitochondrial mRNAs terminate with an intact dodecamer sequence. In light of the recent identification of a protein complex within mitochondria that binds to RNAs terminating with an intact dodecamer sequence, these results support the idea that the dodecamer sequence functions not only within pre-mRNAs as a processing site, but within mature mRNAs as well, possibly for the stabilization and/or translation.

A nucleoside triphosphate-regulated, 3' exonucleolytic mechanism is involved in turnover of yeast mitochondrial RNAs

We have employed cell-free transcription reactions with mitochondria isolated from Saccharomyces cerevisiae to study the mechanism of RNA turnover. The specificity of RNA turnover was preserved in these preparations, as were other RNA-processing reactions, including splicing, 3' end formation of mRNAs, and maturation of rRNAs. Turnover of nascent RNAs was found to occur exonucleolytically; endonucleolytic cleavage products were not detected during turnover of the omega intron RNA, which was studied in detail. However, these experiments still leave open the possibility that endonucleolytic cleavage products with very short half-lives are kinetic intermediates in the decay of omega RNA. Exonucleolytic turnover was regulated by nucleotide triphosphates and required their hydrolysis. A unique signature of this regulation was that any one of the eight standard ribo- or deoxyribonucleotide triphosphates supported RNA turnover. A novel hybrid selection protocol was used to determine the turnover rates of the 5', middle, and 3' portions of one mitochondrial transcript, the omega intron RNA. The results suggested that degradation along that transcript occurred with a 3'-->5' polarity. The similarity between features of mitochondrial RNA turnover and the properties of a nucleotide triphosphate-dependent 3' exoribonuclease that has been purified from yeast mitochondria suggests that this single enzyme is a key activity whose regulation is involved in the specificity of mitochondrial RNA turnover.

Polymorphisms in tandemly repeated sequences of Saccharomyces cerevisiae mitochondrial DNA

A spontaneously arising mitochondrial DNA (mtDNA) variant of Saccharomyces cerevisiae has been formed by two extra copies of a 14-bp sequence (TTAATTAAATTATC) being added to a tandem repeat of this unit. Similar polymorphisms in tandemly repeated sequences have been found in a comparison between mtDNAs from our strain and others. In 5850 bp of intergenic mtDNA sequence, polymorphisms in tandemly repeated sequences of three or more base pairs occur approximately every 400-500 bp whereas differences in 1-2 bp occur approximately every 60 bp. Some polymorphisms are associated with optional G + C-rich sequences (GC clusters). Two such optional GC clusters and one A + T repeat polymorphism have been discovered in the tRNA synthesis locus. In addition, the variable presence of large open reading frames are documented and mechanisms for generating intergenic sequence diversity in S. cerevisiae mtDNA are discussed.

CBP1 function is required for stability of a hybrid cob-oli1 transcript in yeast mitochondria

The nuclear gene product CBP1 stabilizes cytochrome b transcripts in yeast mitochondria. In cbp1 mutant strains, cytochrome b gene (cob) transcripts are not detectable by Northern blot analysis. The results of previous studies led to the hypothesis that CBP1 interacts with the 5'-untranslated sequence of the cob mRNA, or pre-mRNA, to stabilize the message. To determine what portion of the cob leader is sufficient for interaction with CBP1, we have investigated the stability of transcripts from a novel hybrid gene, cob-oli1, in which the 5'-terminal third of the cob leader sequence was fused to the coding sequence of the gene for ATP synthase subunit 9, oli1. The hybrid cob-oli1 transcript was stable in a strain wild-type at the CBP1 locus, but was undetectable in the cbp1 mutant background. That the cob-oli1 transcript was translated to produce ATP synthase subunit 9 in CBP1 strains containing the cob-oli1 gene was verified by 35S-methionine labeling of mitochondrial proteins. We conclude that the 5'-terminal portion of the cob message is sufficient for CBP1 function and discuss the hypothesis that CBP1 interacts directly with this region of the transcript to promote cob mRNA stability.

Protein encoded by the third intron of cytochrome b gene in Saccharomyces cerevisiae is an mRNA maturase. Analysis of mitochondrial mutants, RNA transcripts proteins and evolutionary relationships

We have established the nucleotide sequence of the wild-type and that of a trans-acting mutant located in the third (bi3) intron of the Saccharomyces cerevisiae mitochondrial cytochrome b gene. The intron, 1691 base-pairs long, has an open reading frame 1045 base-pairs long, in phase with the preceding exon and the mutation replaces the evolutionarily conserved Gly codon of the second consensus motif by an Asp codon and blocks the formation of mature cytochrome b mRNA. Splicing intermediates of 5300 and 3900 bases with unexcised bi3 intron and a characteristic novel polypeptide (p50), the size of which corresponds to the chimeric protein encoded by upstream exons and the bi3 intronic open reading frame (ORF), accumulate in this and other bi3 splicing-deficient mutants. We conclude that the protein encoded by the bi3 ORF is a specific mRNA maturase involved in the splicing of the cytochrome b mRNA. The open reading frame of the third intron is remarkably similar to that of the unique intron of the cytochrome b gene (cob A) of Aspergillus nidulans. Both are located in exactly the same position and possibly derive from a recent common ancestor by a horizontal transfer. We have established the nucleotide sequence of an exonic mutant located in the B3 exon. This missense mutation changes the Phe codon 151 into a Cys codon and leads to the absence of functional cytochrome b but does not affect splicing. Finally, we have studied the splicing pathway leading to the synthesis of cytochrome b mRNA by analysing, in a comprehensive manner, the 22 splicing intermediates of several mutants located in bi3.

Expression of the oxi1 and maturase-related RF1 genes in yeast mitochondria

Transcription of the yeast mitochondrial oxi1 gene (cytochrome oxidase subunit 2) is initiated at a variant non anucleotide sequence, TTAAAAGTA, located 54 bp upstream from the protein-coding gene. Transcriptional initiation at this site gives rise to a 2,500 nucleotide primary transcript containing both the oxi1 gene and the downstream maturase-related reading frame, RF1. Precise transcript mapping has revealed that the 3'-end of the mature oxi1 mRNA is generated by an endonucleolytic cleavage which takes place after the conserved dodecamer sequence, AAUAAUAUUCUU (End-of-Messenger signal), 75 nucleotides downstream from the oxi1 stop codon. Since the RF1 5'-terminal coding region overlaps the oxi1 3'-terminal coding sequence, cleavage at this motif truncates the RF1 message suggesting that the expression of the putative RF1 protein is controlled at the level of dodecamer processing.

ARS activity along the yeast mitochondrial apocytochrome b region: correlation with the location of petite genomes and consensus sequences

Seven MboI fragments spanning the mitochondrial apocytochrome b gene in Saccharomyces cerevisiae strain D273-10B were cloned in the BamHI site of the integrative yeast vector YIp5 and the capacity for autonomous replication was subsequently assayed in yeast. The positive correlation found between the ars-like activity in four fragments and the presence of regions common to multiple ethidium bromide-induced petite (rho-) genomes suggests that the mitochondrial sequences possibly active as origins of replication in low-complexity neutral or weakly suppressive rho- mutants could be functionally related to the yeast nuclear replicator 11 nucleotide motif defined by Broach et al. (1983).

The AT spacers and the var1 genes from the mitochondrial genomes of Saccharomyces cerevisiae and Torulopsis glabrata: evolutionary origin and mechanism of formation

Intergenic sequences represent 63% of the mitochondrial 'long' (85 kb) genome of Saccharomyces cerevisiae. They comprise 170-200 AT spacers that correspond to 47% of the genome and are separated from each other by GC clusters, ORFs, ori sequences, as well as by protein-coding genes. Intergenic AT spacers have an average size of 190 bp, and a GC level of 5%; they are formed by short (20-30 nt on the average) A/T stretches separated by C/G mono- to trinucleotides. An analysis of the primary structures of all intergenic AT spacers already sequenced (32 kb; 80% of the total) has shown that they are characterized by an extremely high level of short sequence repetitiveness and by a characteristic sequence pattern; the frequencies of A/T isostichs conspicuously deviate from statistical expectations, and exponentially decrease when their (AT + TA)/(AA + TT) ratio, R, decreases. A situation basically identical was found in the AT spacers of the mitochondrial genome (19 kb) of Torulopsis glabrata. The sequence features of the AT spacers indicate that they were built in evolution by an expansion process mainly involving rounds of duplication, inversion and translocation events which affected an initial oligodeoxynucleotide (endowed with a particular R ratio) and the sequences derived from it. In turn, the initial oligodeoxynucleotide appears to have arisen from an ancestral promoter-replicator sequence which was at the origin of the nonanucleotide promoters present in the mitochondrial genomes of several yeasts. Common sequence patterns indicate that the AT spacers so formed gave rise to the var1 gene (by linking and phasing of short ORFs), to the DNA stretches corresponding to the untranslated mRNA sequences and to the central stretches of ori sequences from S. cerevisiae.

The yeast nuclear gene CBS1 is required for translation of mitochondrial mRNAs bearing the cob 5' untranslated leader

Mitochondrial translation of the cob mRNA to yield apocytochrome b is specifically dependent on the nuclear gene CBS1, while mitochondrial translation of the oxi2 mRNA to yield cytochrome oxidase subunit III (cox III) is specifically dependent on the nuclear gene PET494. Chimeric oxi2 mRNAs bearing the 5' leaders of other mitochondrial mRNAs, transcribed from rho- mitochondrial DNAs termed MSU494, are translated in pet494 mutants. In this study, we examined translation of coxIII from MSU494-encoded chimeric mRNAs in zygotes of defined nuclear and mitochondrial genotype. CoxIII was translated from a chimeric mRNA bearing the cob leader only when the zygotes contained a wild-type CBS1 gene. CoxIII translation from an mRNA bearing the 5' leader of the mitochondrial gene aap1 was not dependent on CBS1 activity. We conclude that the product of the nuclear gene CBS1, or something under its control, acts in the mitochondrion on the cob mRNA 5' leader to activate translation of down-stream coding sequences.

Two yeast nuclear genes, CBS1 and CBS2, are required for translation of mitochondrial transcripts bearing the 5'-untranslated COB leader

Mutations in one of the yeast nuclear genes CBS1 or CBS2 both prevent the excision of the maturase-coding introns bI2, bI3 and bI4 from the mitochondrial COB precursor transcript. Mutant strain MK2 (cbs1-1) has recently been reported to be primarily defective in the translation of COB transcripts, as it can be suppressed by a fusion of the COB structural gene with the 5' untranslated leader of the mitochondrial OLI1 gene (G. Rödel, A. Körte and F. Kaudewitz, Curr Genet 9: 641-648). Here I report that the effect of mutation cbs2-1, too, is suppressed by this gene rearrangement. CBS2 is the second nuclear gene identified which is involved in the translation of mitochondrial transcripts bearing the 5' untranslated COB leader. Gene specific translation control appears to be a major mode of regulation of mitochondrial gene expression in Saccharomyces cerevisiae.

The mosaic cox1 gene in the mitochondrial genome of Schizosaccharomyces pombe: minimal structural requirements and evolution of group I introns

The gene encoding subunit 1 of cytochrome oxidase (cox1) in the fission yeast Schizosaccharomyces pombe is polymorphic. In strain 50 it contains two group I introns with open reading frames (ORFs) in phase with the upstream exons (Lang, 1984). In strain EF1 two additional very short group I introns which do not possess ORFs were detected by DNA sequencing. These two introns (AI2a and AI3) share distinct characteristics concerning their nucleotide sequence and secondary structure and are located at identical positions as the introns AI4 and AI5 beta, respectively, in the cox1 gene of Saccharomyces cerevisiae. The sequence homology of the cob and cox1 genes around the splice points of introns AI2a, AI4, and BI4 (cob intron 4) might reflect horizontal gene transfer between the distantly related species S. pombe and S. cerevisiae.

The primary structure of the mitochondrial genome of Saccharomyces cerevisiae--a review

We have collated and compiled all the available primary structure data on the mitochondrial genome of Saccharomyces cerevisiae. Data concern 78,500 bp, namely 92% of the 'long' genomes; they are derived from several laboratory strains. Interstrain differences belong to three classes: a small number of large deletions/additions, mainly concerning introns; a large number of small (10-150 bp) deletions/additions located in the intergenic sequences; 1-3 bp deletions/additions and point mutations; the interstrain sequence divergence due to the latter, is of the order of 2% for the strains compared; this low value is, however, an overestimate because of sequence mistakes.

Deletions in the cob gene of yeast mtDNA and their phenotypic effect

Two cob- deletion mutants are characterized. One of them, M9410, is deleted for 911 bp of the noncoding sequences only which separate tRNAGlu and cob exon 1; it thus lacks most of the sequence encoding the 957 bp long cob leader (Bonitz et al. 1982) and some 20 bp 5' to it. The end points of this deletion coincide with 31 bp long direct repeats in wild type mtDNA. The other mutant, M9391, is deleted for all cob coding sequences and most of the cob leader sequence but it retains the 5' terminal 261 bp of this leader. Northern analysis revealed that M9410 totally lacks cob mRNA or pre-mRNA. The large deletion M9391 in contrast accumulates a 13S RNA which probably results from transcription through the junction, which ligates sequences of the cob leader to sequences of the cob-oli1 intergenic spacer.

Sequence organization of the mitochondrial genome of yeast--a review

We have compiled the available primary structural data for the mitochondrial genome of Saccharomyces cerevisiae and have estimated the size of the remaining gaps, which represent 12-13% of the genome. The lengths of sequenced regions and of gaps lead to a new assessment of genome sizes; these range (in round figures) from 85 000 bp for the long genomes, to 78 000 bp for the short genomes, to 74 000 bp for the supershort genome of Saccharomyces carlsbergensis. These values are 8-11% higher than those previously estimated from restriction fragments. Interstrain differences concern not only facultative intervening sequences (introns) and mini-inserts, but also insertions/deletions in intergenic sequences. The primary structure appears to be extremely conserved in genes and ori sequences, and highly conserved in intergenic sequences. Since coding sequences represent at most 33-35% of the genome, at least two thirds of the genome are formed by noncoding and yet highly conserved sequences. The G + C level of genes or exon is 25%, and that of intronic open reading frames (ORFs) 22%; increasingly lower values are shown by intronic closed reading frames (CRFs), 20%, ori sequences, 19%, intergenic ORFs, 17.5% and intergenic sequences, 15%.

Mitochondrial suppression of a yeast nuclear mutation which affects the translation of the mitochondrial apocytochrome b transcript

We describe a mitochondrial suppressor mutation, which restores respiratory competence to the nuclear pet- -mutant MK2. This mutant lacks the message of the mitochondrial cob-gene and instead accumulates a partially spliced pre-mRNA which is not translated. Complete processing and translation of the cob-RNA is restored by a rearrangement of the mitochondrial DNA, leading to a fusion of the cob-coding sequences with the leader of oli1, the mitochondrial gene coding for subunit IX of the ATPase. We conclude that the nuclear gene affected in MK2 is essential to allow translation of transcripts which contain the cob-leader sequence.

Molecular basis of the 'box effect', A maturase deficiency leading to the absence of splicing of two introns located in two split genes of yeast mitochondrial DNA

In the mitochondrial DNA of Saccharomyces cerevisiae, the genes cob-box and oxi3, coding for apocytochrome b and cytochrome oxidase subunit I respectively, are split. Several mutations located in the introns of the cob-box gene prevent the synthesis of cytochrome b and cytochrome oxidase subunit I (this is known as the 'box effect').-We have elucidated the molecular basis of this phenomenon: these mutants are unable to excise the fourth intron of oxi3 from the cytochrome oxidase subunit I pre-mRNA; the absence of a functional bI4 mRNA maturase, a trans-acting factor encoded by the fourth intron of the cob-box gene explains this phenomenon. This maturase was already known to control the excision of the bI4 intron; consequently we have demonstrated that it is necessary for the processing of two introns located in two different genes. Mutations altering this maturase can be corrected, but only partially, by extragenic suppressors located in the mitochondrial (mim2) or in the nuclear (NAM2) genome. The gene product of these two suppressors should, therefore, control (directly or indirectly) the excision of the two introns as the bI4 mRNA maturase normally does.

Organization and processing of the mitochondrial oxi3/oli2 multigenic transcript in yeast

In the present article, we confirm our previous proposal (Faye and Simon 1983a, b) that the oxi3 and oli2 genes belong to the same transcription unit. Furthermore, we have shown that a primary polycistronic transcript covers oxi3, aap1, oli2 and extends beyond URF2. Transcriptional analysis of this region revealed several cleavage points. The examination of the DNA sequence at and surrounding these cleavage points disclosed that some of them take place at or near specific sequences found also in other known multigenic transcripts. Two of the major cleavages involve the stem-loop structure of GC rich clusters. We discuss the possibility that some of these cleavage sites serve as post-transcriptional processing signals and may be necessary for the maturation of the precursor RNA.

A rapid method for detecting specific RNA transcripts by hybridization to DNA probes in solution

A method is described for detecting specific transcripts in crude mixtures of RNA. The method employs hybridization of single-stranded or double-stranded radioactive DNA probes in solution, followed by electrophoretic separation of the hybrid and probe on agarose and visualization by radioautography. The procedure offers the advantages of decreased preparation time and increased sensitivity over currently used methods.

Genetics and biogenesis of cytochrome b

We have reviewed here the genetic methods used for isolating and manipulating nuclear and mitochondrial mutants of bakers' yeast that affect the function and biogenesis of complex III of the mitochondrial respiratory chain. All the methods have been used with success in the past, and it is hoped that this compilation will aid biochemists in using these techniques to study electron transfer.