Cytochrome oxidase subunit V gene of Neurospora crassa: DNA sequences, chromosomal mapping, and evidence that the cya-4 locus specifies the structural gene for subunit V

The sequences of cDNA and genomic DNA clones for Neurospora cytochrome oxidase subunit V show that the protein is synthesized as a 171-amino-acid precursor containing a 27-amino-acid N-terminal extension. The subunit V protein sequence is 34% identical to that of Saccharomyces cerevisiae subunit V; these proteins, as well as the corresponding bovine subunit, subunit IV, contain a single hydrophobic domain which most likely spans the inner mitochondrial membrane. The Neurospora crassa subunit V gene (cox5) contains two introns, 398 and 68 nucleotides long, which share the conserved intron boundaries 5'GTRNGT...CAG3' and the internal consensus sequence ACTRACA. Two short sequences, YGCCAG and YCCGTTY, are repeated four times each in the cox5 gene upstream of the mRNA 5' termini. The cox5 mRNA 5' ends are heterogeneous, with the major mRNA 5' end located 144 to 147 nucleotides upstream from the translational start site. The mRNA contains a 3'-untranslated region of 186 to 187 nucleotides. Using restriction-fragment-length polymorphism, we mapped the cox5 gene to linkage group IIR, close to the arg-5 locus. Since one of the mutations causing cytochrome oxidase deficiency in N. crassa, cya-4-23, also maps there, we transformed the cya-4-23 strain with the wild-type cox5 gene. In contrast to cya-4-23 cells, which grow slowly, cox5 transformants grew quickly, contained cytochrome oxidase, and had 8- to 11-fold-higher levels of subunit V in their mitochondria. These data suggest (i) that the cya-4 locus in N. crassa specifies structural information for cytochrome oxidase subunit V and (ii) that, in N. crassa, as in S. cerevisiae, deficiencies in the production of nuclearly encoded cytochrome oxidase subunits result in deficiency in cytochrome oxidase activity. Finally, we show that the lower levels of subunit V in cya-4-23 cells are most likely due to substantially reduced levels of translatable subunit V mRNA.

Cytoplasmic leucyl-tRNA synthetase of Neurospora crassa is not specified by the leu-5 locus

We generated a lambda gt11 Neurospora crassa cDNA library and screened the library for the cytoplasmic leucyl-tRNA synthetase (cyto LeuRS) clones using cyto LeuRS specific antibody. Two clones, lambda NCLRSC1 and lambda NCLRSC2, were obtained which have inserts of approximately 2 kbp and approximately 1.3 kbp, and which overlap by about 0.6 kbp. The following lines of evidence indicate that lambda NCLRSC1 and lambda NCLRSC2 encode parts of cyto LeuRS. (1) Antibodies affinity purified using either of the fusion proteins encoded by lambda NCLRSC1 or lambda NCLRSC2 inhibit cyto LeuRS activity. Thus, the fusion protein and cyto LeuRS share immunological determinants. (2) The same antibodies also react with an approximately 115-kDa protein, which comigrates with purified cyto LeuRS, in immunoblots of total N. crassa proteins. We used the cDNA clones to probe a N. crassa genomic DNA library and isolated two genomic DNA clones. Partial sequence analysis of cDNA and genomic DNA clones shows a methionine initiated open reading frame, which includes a stretch of amino acid residues that are highly conserved and that are at the ATP binding site in aminoacyl-tRNA synthetases. Using the cloned DNA as probe, we show that the cyto LeuRS mRNA is approximately 3900 nucleotides long. Finally, we have used restriction fragment length polymorphism mapping to show that the cyto LeuRS gene resides on the far right of linkage group II and not on linkage group V where the leu-5 mutation, which was previously reported to specify cyto LeuRS, is located.

The recognition by RNase P of precursor tRNAs

We have generated mutants of M1 RNA, the catalytic subunit of Escherichia coli RNaseP, and have analyzed their properties in vitro and in vivo. The mutations, A333----C333, A334----U334, and A333 A334----C333 U334 are within the sequence UGAAU which is complementary to the GT psi CR sequence found in loop IV of all E. coli tRNAs. We have examined: 1) whether the mutant M1 RNAs are active in processing wild type tRNA precursors and 2) whether they can restore the processing defect in mutant tRNA precursors with changes within the GT psi CR sequence. As substrates for in vitro studies we used wild type E. coli SuIII tRNA(Tyr) precursor, and pTyrA54, a mutant tRNA precursor with a base change that could potentially complement the U334 mutation in M1 RNA. The C333 mutation had no effect on activity of M1 RNA on wild type pTyr. The U334 mutant M1 RNA, on the other hand, had a much lower activity on wild type pTyr. However, use of pTyrA54 as substrate instead of wild type pTyr did not restore the activity of the U334 mutant M1 RNA. These results suggest that interactions via base pairing between nucleotides 331-335 of M1 RNA and the GT psi CG of pTyr are probably not essential for cleavage of these tRNA precursors by M1 RNA. For assays of in vivo function, we examined the ability of mutant M1 RNAs to complement a ts mutation in the protein component of RNaseP in FS101, a recA- derivative of E. coli strain A49. In contrast to wild type M1 RNA, which complements the ts mutation when it is overproduced, neither the C333 nor the U334 mutant M1 RNAs was able to do so.

Saccharomyces cerevisiae SUP53 tRNA gene transcripts are processed by mammalian cell extracts in vitro but are not processed in vivo

We describe the results of our studies of expression of a Saccharomyces cerevisiae amber suppressor tRNA(Leu) gene (SUP53) in mammalian cells in vivo and in cell extracts in vitro. Parallel studies were carried out with the wild-type (Su-) tRNA(Leu) gene. Extracts from HeLa or CV1 cells transcribed both tRNA(Leu) genes. The transcripts were processed correctly at the 5' and 3' ends and accurately spliced to produce mature tRNA(Leu). Surprisingly, when the same tRNA(Leu) genes were introduced into CV1 cells, only pre-tRNAs(Leu) were produced. The pre-tRNAs(Leu) made in vivo were of the same size and contained the 5'-leader and 3'-trailer sequences as did pre-tRNAs(Leu) made in vitro. Furthermore, the pre-tRNAs(Leu) made in vivo were processed to mature tRNA(Leu) when incubated with HeLa cell extracts. A tRNA(Leu) gene from which the intervening sequence had been removed yielded RNAs that also were not processed at either their 5' or 3' termini. Thus, processing of pre-tRNA(Leu) in CV1 cells is blocked at the level of 5'- and 3'-end maturation. One possible explanation of the discrepancy in the results obtained in vivo and in vitro is that tRNA biosynthesis in mammalian cells involves transport of pre-tRNA from the site of its synthesis to a site or sites where processing takes place, and perhaps the yeast pre-tRNAs(Leu) synthesized in CV1 cells are not transported to the appropriate site.

Mutants of Escherichia coli formylmethionine tRNA: a single base change enables initiator tRNA to act as an elongator in vitro

We show that the absence of a Watson-Crick base pair at the end of the amino acid acceptor stem, which is a hallmark of all prokaryotic initiator tRNAs, is one of the key features that prevents them from acting as an elongator in protein synthesis. We generated mutants of Escherichia coli formylmethionine tRNA that have a base pair at the end of the acceptor stem. The mutants generated were C1----T1, which had a U.A base pair, A72----G72, which had a C.G base pair, and the C1A72----T1G72 double mutant, which lacked the base pair. After aminoacylation, the activity of these and other mutant initiator methionyl-tRNAs (Met-tRNAs) in elongation were assayed in a MS2 RNA-directed E. coli protein-synthesizing system and in binding to the elongation factor Tu (EF-Tu). Unlike wild-type initiator tRNA or the T1G72 double mutant, the T1 and G72 mutant Met-tRNAs were active in elongation, the G72 mutant being more active than the T1 mutant. The T1 and G72 mutant Met-tRNAs also formed a ternary complex with elongation factor EF-Tu.GTP, and their relative affinities for EF-Tu.GTP paralleled their activities in elongation. Combination of the T1 or G72 mutation with mutations in the GGG.CCC sequence conserved in the anticodon stem of initiator tRNAs led to a further increase in the activities of these mutant tRNAs in elongation such that one of these mutants was now almost as good an elongator as E. coli elongator methionine tRNA.

An inducible mammalian amber suppressor: propagation of a poliovirus mutant

We describe a general protocol for controlled gene amplification, which allows conditional expression of high levels of amber suppressor activity in monkey kidney cells, and we demonstrate its use in the genetic analysis of animal viruses by the generation and propagation of the first nonsense mutant of poliovirus. A human amber suppressor tRNASer gene linked to the SV40 origin of replication and a second DNA carrying a temperature-sensitive SV40 large T antigen gene were cotransfected into monkey cells. Cell lines having stably integrated the DNAs were isolated. Shifting the cells from the nonpermissive temperature to a lower permissive temperature caused the amplification of the suppressor tRNA gene, which resulted in suppression efficiencies at amber codons of 50%-70%, as measured by suppression of an amber codon in the E. coli chloramphenicol acetyltransferase gene. A mutant of poliovirus, in which a serine codon in the replicase gene was converted to an amber codon, was efficiently propagated on the suppressor-positive cell lines. The mutant virus reverted to wild-type by a single base change to a serine codon at a frequency of approximately 2.5 x 10(-6), surprisingly low for a RNA genome.

Escherichia coli formylmethionine tRNA: mutations in GGGCCC sequence conserved in anticodon stem of initiator tRNAs affect initiation of protein synthesis and conformation of anticodon loop

We have generated mutants of Escherichia coli formylmethionine initiator tRNA in which one, two, and all three G X C base pairs in the GGGCCC sequence in the anticodon stem are changed to those found in E. coli elongator methionine tRNA. Overproduction of the mutant tRNAs using M13 recombinants as an expression vector and development of a one-step purification scheme allowed us to purify, characterize, and analyze the function of the mutant tRNAs. After aminoacylation and formylation, the function of mutant formylmethionyl tRNAs was analyzed in an MS2 RNA-directed in vitro protein-synthesizing system, in AUG-dependent ribosomal P site binding, and in initiation factor binding. The mutant tRNAs show progressive loss of activity in initiation, the mutant with all three G X C base pairs substituted being the least active. The mutations affect binding to the ribosomal P site. None of the mutations affects binding to initiation factor 2. We also show that there is a progressive increase in accessibility of phosphodiester bonds in the anticodon loop of the three mutants to S1 nuclease, such that the cleavage pattern of the mutant with all three G X C base-pair changes resembles that of elongator tRNAs. These results are consistent with the notion that the contiguous G X C base pairs in the anticodon stem of initiator tRNAs impart on the anticodon loop a unique conformation, which may be important in targeting the initiator tRNA to the ribosomal P site during initiation of protein synthesis.

Introduction of UAG, UAA, and UGA nonsense mutations at a specific site in the Escherichia coli chloramphenicol acetyltransferase gene: use in measurement of amber, ochre, and opal suppression in mammalian cells

We have used oligonucleotide-directed site-specific mutagenesis to convert serine codon 27 of the Escherichia coli chloramphenicol acetyltransferase (cat) gene to UAG, UAA, and UGA nonsense codons. The mutant cat genes, under transcriptional control of the Rous sarcoma virus long terminal repeat, were then introduced into mammalian cells by DNA transfection along with UAG, UAA, and UGA suppressor tRNA genes derived from a human serine tRNA. Assay for CAT enzymatic activity in extracts from such cells allowed us to detect and quantitate nonsense suppression in monkey CV-1 cells and mouse NIH3T3 cells. Using such an assay, we provide the first direct evidence that an opal suppressor tRNA gene is functional in mammalian cells. The pattern of suppression of the three cat nonsense mutations in bacteria suggests that the serine at position 27 of CAT can be replaced by a wide variety of amino acids without loss of enzymatic activity. Thus, these mutant cat genes should be generally useful for the quantitation of suppressor activity of suppressor tRNA genes introduced into cells and possibly for the detection of naturally occurring nonsense suppressors.

The E35 stopper mutant of Neurospora crassa: precise localization of deletion endpoints in mitochondrial DNA and evidence that the deleted DNA codes for a subunit of NADH dehydrogenase

Two types of defective mitochondrial DNA molecules with large deletions (5 kbp and 40 kbp) have previously been identified in the stopper mutant, E35, of Neurospora crassa. The junction fragments spanning the deletion endpoints have now been cloned and sequenced, and their sequences compared with those of the corresponding wild-type fragments. We show that both types of defective mitochondrial DNAs result from deletions of sequences flanked by short direct repeats, which are themselves parts of larger inverted repeat sequences. In every case, the short direct repeat sequences consist of a run of pyrimidines in one strand and purines in the other. We also report the sequence of a 2151-bp HindIII fragment, which is deleted in both of the defective mitochondrial DNAs. Besides the previously identified gene for a methionine tRNA, the 2151-bp DNA sequence contains an open reading frame with the potential to code for a hydrophobic protein 583 amino acids long. This hydrophobic protein has three blocks of significant homology with proteins coded by URF2 found in other mitochondrial genomes. Since the mammalian mitochondrial URF2 has recently been shown to code for a subunit of NADH dehydrogenase, part of the DNA sequence missing in the E35 stopper mutant of N. crassa may also code for a subunit of NADH dehydrogenase.

A single mutation in loop IV of Escherichia coli SuIII tRNA blocks processing at both 5'- and 3'-ends of the precursor tRNA

We showed previously that a single mutation (T54----A54) within the highly conserved GT psi C sequence in the Escherichia coli SuIII tRNA gene results in an Su- phenotype, and we presented preliminary evidence that this mutation affected biosynthesis of the tRNA. We now show that the A54 mutation has no effect on transcription but prevents accumulation of mature mutant tRNA. The absence of mature mutant tRNA is due to blocks in processing of the precursor tRNA and is not due to instability of the mature tRNA. Characterization of the A54 precursors (approximately 130, approximately 90, and approximately 85 nucleotides long), which accumulate in E. coli minicells, indicates multiple blocks to processing of mutant transcripts. All three size classes of precursors have heterogeneous 3'-termini and contain extensions of three to seven nucleotides beyond the 3'-CCA sequence of mature tRNA indicating either a block or delay in 3'-exonucleolytic processing. The approximately 130- and approximately 90-nucleotide-long precursors also have 5'-terminal extensions suggesting a block in the 5'-processing reaction catalyzed by RNase P. Only the approximately 85-nucleotide-long precursor, which represents a small fraction of the total tRNA precursors, has the correct 5'-end of the mature tRNA.

Nuclear genes for cytochrome c oxidase subunits of Neurospora crassa. Isolation and characterization of cDNA clones for subunits IV, V, VI, and possibly VII

We obtained cDNA clones for cytochrome oxidase subunits IV, V, VI, and possibly VII by constructing a lambda gt11 library of Neurospora crassa cDNA and probing it with antiserum directed against Neurospora cytochrome oxidase holoenzyme. Positive clones were further characterized with antisera directed against individual cytochrome oxidase subunits and subsequently by DNA sequencing. The clones for subunits IV and V encode proteins with regions matching the known N-terminal amino acid sequences of purified Neurospora cytochrome oxidase subunits IV and V, respectively. The sequences of these clones provide the first evidence that cytochrome oxidase subunits IV and V are made as precursors with N-terminal extensions in Neurospora. The N-terminal extensions encoded by these clones share homology, and are rich in arginine, as are signal sequences of other mitochondrially destined proteins. The subunit VI clone codes for the carboxyl terminus of a protein homologous to the carboxy termini of yeast cytochrome oxidase subunit VI and bovine cytochrome oxidase subunit Va. The subunit VII clone contains an open reading frame for a 47-residue protein, the expected size for subunit VII. However, the protein coded by this clone has an unusual amino acid composition. Whether this clone represents an authentic cytochrome oxidase subunit is not established.

Attempted expression of a human initiator tRNA gene in Saccharomyces cerevisiae

In attempts to overproduce the wild type and, eventually, mutant human initiator methionine tRNAs for use in structure-function relationship studies, we have investigated the expression of the wild type human initiator tRNA gene in the yeast Saccharomyces cerevisiae, both in vitro and in vivo. We find that the yeast extract, while capable of accurately transcribing several yeast tRNA genes, does not transcribe the human initiator tRNA gene. In addition, when the human initiator tRNA gene is introduced into yeast as part of a 2 mu vector, no expression of the human tRNA gene was detected. A yeast alanine tRNA gene similarly introduced into yeast is expressed efficiently. The block in expression of the human tRNA gene is at the level of transcription and not processing. The yeast cell-free extract can accurately process precursors of the same human initiator tRNA made in a HeLa cell-free extract. Surprisingly, although the human tRNA gene has essentially the same intragenic control elements as the yeast initiator tRNA gene, the human tRNA gene competes extremely poorly for transcription factors in yeast extracts. In the course of screening a yeast DNA bank for initiator tRNA clones we have isolated and sequenced three yeast tRNA genes corresponding to glycine, alanine, and aspartic acid tRNAs. The sequence of glycine tRNA gene differs from the published tRNA sequence in having an additional nucleotide in the variable loop. The alanine tRNA gene codes for a new tRNA. All three genes are transcriptionally active in yeast extracts.

Site-specific mutagenesis on a human initiator methionine tRNA gene within a sequence conserved in all eukaryotic initiator tRNAs and studies of its effects on in vitro transcription

We have used oligonucleotide-directed site-specific mutagenesis to generate a mutant human initiator tRNA gene in which the sequence GATCG corresponding to the universal GAUCG found in loop IV of eukaryotic cytoplasmic initiator tRNAs is changed to GTTCG. The mutant tRNA gene has been characterized by restriction mapping and by DNA sequencing. We show that this mutation has no effect on in vitro transcription of the tRNA gene in HeLa cell extracts. Transcripts derived from both the wild type (A54) and the mutant (T54) initiator tRNA genes are processed in vitro to produce mature tRNAs with the correct 5'-and 3'-termini. Fingerprint analysis of in vitro transcripts shows that the mutant RNA has the expected nucleotide change. Modified nucleotide composition analyses on the RNAs show that when A54 is changed to U54, the neighboring nucleotide U55 is modified quantitatively to psi 55 in the in vitro extracts; U54 itself is partially modified to ribo-T. Other modified bases identified in the in vitro transcripts include m1G, m2G, m7G, D, and m5C.

Expression in vivo of a mutant human initiator tRNA gene in mammalian cells using a simian virus 40 vector

We have cloned both the wild type (A54) and mutant (T54) human initiator genes described in the preceding paper (Drabkin, H. J., and RajBhandary, U. L. (1985) J. Biol. Chem. 260, 5580-5587) as 141-base pair fragments into the SV40-pBR322 vector pSV1GT3. These vectors were subsequently used to transfect monkey kidney CV-1 cells to obtain recombinant virus stocks carrying each of the initiator tRNA genes. Following infection of CV-1 cells by the recombinant virus stocks, both the wild type and mutant tRNAs are produced in large quantities during a 48-h period. Fingerprint analysis of 32P-labeled tRNAs was used to characterize the tRNAs made in vivo and to show that the sequence AUCG in loop IV of the wild type tRNA is replaced by T psi CG in the mutant tRNA. Modified nucleotide composition analysis of the [32P]tRNAs overproduced in vivo shows that they contain all the modified nucleotides found in human placenta initiator tRNA. Both wild type and mutant initiator tRNAs can be aminoacylated by either mammalian or Escherichia coli methionyl-tRNA synthetases. Furthermore, the mutant tRNA can be easily separated from the endogenous monkey initiator tRNA by RPC-5 column chromatography.

Amber, ochre and opal suppressor tRNA genes derived from a human serine tRNA gene

Amber, ochre and opal suppressor tRNA genes have been generated by using oligonucleotide directed site-specific mutagenesis to change one or two nucleotides in a human serine tRNA gene. The amber and ochre suppressor (Su+) tRNA genes are efficiently expressed in CV-1 cells when introduced as part of a SV40 recombinant. The expressed amber and ochre Su+ tRNAs are functional as suppressors as demonstrated by readthrough of the amber codon which terminates the NS1 gene of an influenza virus or the ochre codon which terminates the hexon gene of adenovirus, respectively. Interestingly, several attempts to obtain the equivalent virus stock of an SV40 recombinant containing the opal suppressor tRNA gene yielded virus lacking the opal suppressor tRNA gene. This suggests that expression of an efficient opal suppressor derived from a human serine tRNA gene is highly detrimental to either cellular or viral processes.

RNA processing in Neurospora crassa mitochondria: use of transfer RNA sequences as signals

We have used RNA gel transfer hybridization, S1 nuclease mapping and primer extension to analyze transcripts derived from several genes in Neurospora crassa mitochondria. The transcripts studied include those for cytochrome oxidase subunit III, 17S rRNA and an unidentified open reading frame. In all three cases, initial transcripts are long, include tRNA sequences, and are subsequently processed to generate the mature RNAs. We find that endpoints of the most abundant transcripts generally coincide with those of tRNA sequences. We therefore conclude that tRNA sequences in long transcripts act as primary signals for RNA processing in N. crassa mitochondria. The situation is somewhat analogous to that observed in mammalian mitochondrial systems. The difference, however, is that in mammalian mitochondria, noncoding spacers between tRNA, rRNA and protein genes are very short and in many cases non-existent, allowing no room for intergenic RNA processing signals whereas, in N. crassa mtDNA, intergenic non-coding sequences are usually several hundred nucleotides long and contain highly conserved GC-rich palindromic sequences. Since these GC-rich palindromic sequences are retained in the processed mature RNAs, we conclude that they do not serve as signals for RNA processing.

Synthesis of an ochre suppressor tRNA gene and expression in mammalian cells

We have used site-specific mutagenesis to change the anticodon of a Xenopus laevis tyrosine tRNA gene so that it would recognize ochre codons. This tRNA gene is expressed when amplified in monkey cells as part of a SV40 recombinant and efficiently suppresses termination at both the ochre codon separating the adenovirus 2 hexon gene from a 23-kd downstream gene and the ochre codon at the end of the NS1 gene of influenza virus A/Tex/1/68. Termination at an amber codon of a NS1 gene of another influenza virus strain was not suppressed by the (Su+) ochre gene suggesting that in mammalian cells amber codons are not recognized by ochre suppressor tRNAs. Finally, microinjection into mammalian cells of both (Su+) ochre tRNA genes and selectible genes containing ochre nonsense mutations gives rise to colonies under selective conditions. We conclude that it should be possible to isolate a wide assortment of mammalian cell lines with ochre suppressor activity.

Cytochrome b gene of Neurospora crassa mitochondria. Partial sequence and location of introns at sites different from those in Saccharomyces cerevisiae and Aspergillus nidulans

We have sequenced a 2614-base pair fragment of the Neurospora crassa mitochondrial DNA which contains part of the structural gene for apocytochrome b. This gene is split by at least two introns. The sequence reported here begins within one intron, extends through the next exon, another intron 1276 base pairs long, and the last exon which encodes the COOH terminus of cytochrome b. Within the 254 amino acids encoded by the two exons, there is a high degree of sequence conservation, 81%, with cytochrome b of Aspergillus nidulans. Surprisingly, both introns in the N. crassa cytochrome b gene are located at positions different from introns in the corresponding genes in Saccharomyces cerevisiae or A. nidulans. The upstream intron is located 22 nucleotides before the first intron in the long form of the S. cerevisiae cytochrome b gene. The downstream intron is located 16 nucleotides before the third intron in the long form of the S. cerevisiae gene and the only intron in the A. nidulans cytochrome b gene. The 1276-base pair downstream intron contains a 314 amino acid long open reading frame, which is in-phase with the preceding exon. The protein product of this reading frame has some resemblance to intron-encoded proteins, known as "mRNA maturases," which are thought to participate in RNA splicing in the mitochondria of S. cerevisiae. Another feature shared by the downstream intron and most other mitochondrial introns is the presence of the Box 9 and Box 2 consensus sequences, which may also be important for RNA splicing.