Human immunodeficiency virus env expression becomes Rev-independent if the env region is not defined as an intron.

The human immunodeficiency virus (HIV) Rev protein functions to facilitate export of intron-containing HIV mRNA from the nucleus to the cytoplasm. We have previously shown that splice site recognition plays an important role in Rev regulation of HIV env expression. Here we have further analyzed the effects of splice sites on HIV env expression and Rev regulation, using a simian virus 40 late replacement vector system. env expression from the vector became completely Rev-independent when an excisable intron was positioned upstream of the env region, provided that env was not recognized as an intron. Complete Rev regulation was restored either by the insertion of a 5' splice site between the intron and the env open reading frame or by deletion of the 3' splice site of the upstream intron. These results show that 5' splice sites can function as cis-acting repressor sequence (CRS) elements to retain RNA in the nucleus in the absence of Rev. They also indicate that Rev regulation of HIV env expression is critically dependent on whether the env region is defined as an intron. This strengthens the hypothesis that Rev interacts with components of the splicing machinery to release splicing factors and enable export of the mRNA before splicing occurs.     

A simple technique for the isolation of deletion mutants of phage lambda.

We describe a simple technique for isolating deletion mutants of phage lambda and use it to dissect a cloned fragment of foreign DNA. The technique is based on our previous finding that the normally essential product of lambda head gene D is dispensible for phage growth if the DNA content of the phage is less than 82% that of lambda wild-type (Sternberg and Weisberg, 1977). A significant fraction of the few phage that form plaques when a D amber mutant is plated on a nonsuppressing host contains deletions that reduce the phage chromosome size to less than 82% that of wild-type. It is possible to isolate deletions ranging in size from less than 1.5 kb to 14 kb (3 to 27% of wild-type lambda), and the size range can be restricted by an appropriate choice of the DNA content of the starting phage. This method, unlike the older EDTA or heat resistance methods, permits the scoring of deletions because of the absence of phenotypic variants. We investigated the effect of several host and phage mutations on deletion frequency and type and have determined that a host polA mutation increases the frequency of deletions about 30-50-fold without changing the type of deletions. A host mutD mutation or thymine deprivation increases deletion frequency about 10-fold. In contrast, a host ligts mutation has no effect on the frequency of deletions. We have also determined that the size of the smallest lambda chromosome packageable in a plaque-forming phage particle is 72-73% that of lambda wild-type.     

Molecular causes of thalassemia. IV. Cloning of the beta-globin gene in a patient with beta-thalassemia from Azerbaijan and determination of the point mutation in a minor intron

On the basis of DNA from a beta-thalassemic patient, human gene library has been obtained using bacteriophage lambda EMBL4 as a vector. The recombinants contain human DNA insertions of 15 to 20 kb. The lambda A1 beta 1 clone has been isolated by the method of hybridization of phage plaque replicas to the HhaI fragment of JW102 plasmid containing human beta-globin cDNA. Restriction mapping revealed the presence of a 22 kb human DNA fragment comprising a portion of the beta-globin gene system. Subcloned fragments of beta-globin gene (within the pUC19 plasmid or phage MI3mp10) were sequenced using the Maxam and Gilbert method as well as that of Sanger. 2150 nucleotides in total were analysed. We have detected the point substitution G----A in the 110 nucleotide of minor intron, leading to the formation of an additional splicing site.     

The DNA between Rz and cosR in bacteriophage lambda is nonessential

Near the right end of phage lambda DNA, between gene Rz and the cos site, are 2050 bp of apparently non-coding DNA. We have cloned a lambda DNA fragment containing this DNA into a plasmid and constructed a deletion, omega l, extending from a site within the Rz gene to a site about 560 bp from cos. This deletion could be recombined into viable lambda phage at a frequency equal to that observed for the undeleted sequence. Recombinant phage lambda carrying the omega l deletion were demonstrated to have the same burst size and kinetics of phage production as undeleted lambda. The omega l deletion can be used to extend the capacity of lambda cloning vectors and to provide a region for the insertion of heterologous DNA which should exhibit controllable high level expression from the lambda late promoter, p'R.     

5''-3'' exonucleases in phosphorothioate-based oligonucleotide-directed mutagenesis.

The application of T7 and lambda exonuclease to phosphorothioate-based oligonucleotide-directed mutagenesis was investigated. Oligonucleotide primers designed to introduce single or double base mismatches, an insertion or a deletion (each of 16 bases) were annealed to M13 phage derivatives. Double stranded closed circular DNA (RF IV) containing phosphorothioate internucleotidic linkages in the (-)strand was prepared enzymatically from these templates. A nick was introduced into the (+)strand of the hetroduplex DNA. This nicked DNA (RF II) was subjected to treatment with T7 or lambda exonuclease. Both of these enzymes were able to degrade almost all of the viral (+)strand when presented with DNA containing one or two base mismatches. Repolymerisation of the DNA after the gapping reaction, followed by transfection into E. coli cells gave mutational efficiencies of up to 95%. In the case of RF II DNA prepared with insertion or deletion primers these exonucleases could only partially degrade the viral (+)strand but were nevertheless highly efficient in such mutagenesis experiments.     

The human thyroglobulin gene is over 300 kb long and contains introns of up to 64 kb.

Thyroglobulin (Tg), the precursor of thyroid hormones, is a 660.000 Da dimeric glycoprotein synthesized exclusively in the thyroid gland. We have cloned the human thyroglobulin gene from cosmid and phage libraries and constructed a complete restriction map. The gene encodes an 8.7 kb mRNA, covers at least 300 kb DNA and contains at least 37 exons separated by large introns of up to 64 kb. A striking difference in structure between the 5' and 3' part of the gene suggests that it is composed of two evolutionarily different regions. The first 30 kb DNA encode 3 kb of the mRNA, yielding an exon:intron ratio of 1:10, whereas the remaining 270 kb encodes 5.7 kb of the mRNA with an exon:intron ratio of 1:47. In thyroid cells, the Tg gene is not rearranged and nuclear RNA homologous with sequences internal to the 64 kb intron is present, suggesting that the Tg gene is transcribed as a 300 kb RNA.     

Transduction of c-src coding and intron sequences by a transformation-defective deletion mutant of Rous sarcoma virus.

The mechanism of cellular src (c-src) transduction by a transformation-defective deletion mutant, td109, of Rous sarcoma virus was studied by sequence analysis of the recombinational junctions in three td109-derived recovered sarcoma viruses (rASVs). Our results show that two rASVs have been generated by recombination between td109 and c-src at the region between exons 1 and 2 defined previously. Significant homology between td109 and c-src sequences was present at the sites of recombination. The viral and c-src sequence junction of the third rASV was formed by splicing a cryptic donor site at the 5' region of env of td109 to exon 1 of c-src. Various lengths of c-src internal intron 1 sequences were incorporated into all three rASV genomes, which resulted from activation of potential splice donor and acceptor sites. The incorporated intron 1 sequences were absent in the c-src mRNA, excluding its being the precursor for recombination with td109 and implying that initial recombinations most likely took place at the DNA level. A potential splice acceptor site within the incorporated intron 1 sequences in two rASVs was activated and was used for the src mRNA synthesis in infected cells. The normal env mRNA splice acceptor site was used for src mRNA synthesis for the third rASV.     

Retroviruses as mutagens: Insertion and excision of a nontransforming provirus alter expression of a resident transforming provirus

Integration of retroviral DNA appears to occur randomly in host genomes, suggesting that retroviruses can act as insertion mutagens. We have confirmed this prediction by showing that the nontransforming retrovirus, Moloney murine leukemia virus (M-MuLV), can insert its provirus within the selectable target provided by a single provirus in a clonal rat cell line (B31) transformed by Rous sarcoma virus (RSV). Analysis of over 60 morphological revertants of M-MuLV-superinfected B31 cells revealed two lines with inserts of M-MuLV proviruses within the RSV provirus but outside the transforming gene of RSV (src), at sites 0.6 and 4.0 kb from the 5′ end. The inserts did not inactivate initiation of RSV RNA synthesis but did affect elongation or processing, or both, generating species with the 5′ end of RSV RNA linked to sequences that presumably derive from the inserted M-MuLV DNA. In one mutant line, most of the insert was excised at low frequency, apparently by homologous recombination between repeated sequences at the ends of M-MuLV DNA. After excision, RSV src mRNA was present in normal amounts, and the cells resumed a transformed appearance. In at least four independent lines, large portions of the left end of the RSV provirus (from 1 to 6 kb) and variable amounts of leftward flanking cellular DNA (from 0.5 to 10–15 kb or more) were deleted, without nearby insertions of M-MuLV DNA. The deletions removed the putative promoter for synthesis of RSV RNA; in the two cases examined, no RSV RNA was detected. These deletions may represent a second mutational effect of the superinfection by M-MuLV.     

Deletions of specific regions of the simian sarcoma-associated virus genome are found in defective viruses and in the simian sarcoma virus.

Extrachromosomal DNA was isolated from tissue culture cells that were acutely infected with simian sarcoma virus (SSV) and its associated helper (simian sarcoma-associated virus [SSAV]). Two sizes of closed circular viral genomic DNA intermediates were isolated, cleaved at the single EcoRI site, and ligated to the Charon 21A phage lambda vector. Cloned molecules of the larger size all represented the full-length (9.0-kilobase [kb]) SSAV molecule. A heterogeneous group of clones was derived from the smaller DNA circles. These included the SSV genome and SSAV deletion mutants. When two SSV clones were compared with the helper, they contained the following three characteristic deletions: (i) a 250-base pair deletion in the gag gene about 1.0 kb from the 5' end of the genome; (ii) a 1.85-kb deletion in the pol gene; and (iii) a 1.9-kb deletion at the 3' end, which included part of the env gene. This latter deletion was the site of the onc gene substitution. Six other clones of the smaller molecules represented the following variants of the SSAV genome: (i) two clones of the entire genome containing only one long terminal repeat unit; (ii) one clone with the 1.85-kb deletion of the pol gene observed in SSSV; and (iii) three clones having a deletion of the 3' end of the SSAV genome. In each of the latter clones, the 5' border of the deletion was indistinguishable from the 5' border of the onc substitution in SSV. The fidelity of genetic deletions observed suggested that certain regions of the SSAV genome were deleted at a high frequency. In certain cases, these deletions may have been accompanied by a substitution of cellular sequences to generate SSV.     

Construction and study of a new type of phage lambda DNA vector molecule]

A group of lambda mutants (mutants lambda 0) harbouring lesser number of EcoRI restriction sites on DNA molecules was selected. lambda3-1 recombinant (genotype lambdab221amgamma210Sr1lambda3+c-Px) was created by crosses of lambda02 phage with other lambda mutants. This phage DNA may be used as a vector molecule which makes it possible to select easily phages harbouring insertions of EcoRI DNA fragments. The maximal size of DNA fragment, the insertion of which would not decrease lambda3-1 viability, is 7.7 megadaltone. Lambda3-1 DNA has three regions heterological to lambda DNA, two of which probably include sites SRIlambda4 and SRIlambda5 and some juxtaposed genes. For example, Ptgene of lambda phage in juxtaposition with site SRIlambda4 is substituted by Px gene on the lambda3-1 DNA molecule.     

Electron microscope heteroduplex studies of sequence relations among bacterial plasmids: identification and mapping of the insertion sequences IS1 and IS2 in F and R plasmids.

Heteroduplex experiments between the plasmid R6 and one strand of the deoxyribonucleic acid (DNA) of a lambda phage carrying the insertion sequence IS1 show that IS1 occurs on R6 at the two previously mapped junctions of resistance transfer factor (RTF) DNA with R-determinant DNA. From previous heteroduplex experiments, it then follows that IS1 occurs at the same junctions in R6-5, R100-1, and R1 plasmids. Heteroduplex experiments with the DNA from a lambda phage carrying the insertion sequence IS2 show that one copy of IS2 occurs in R6, R6-5, and R100-1 (but not R1) at a point within the RTF with coordinates 67.5 TO 68.9 kilobase units (kb). In an accompanying paper, Ptashne and Cohen (1975) show that the insertion sequence IS3 occurs on R6 and R6-5. R100-25, a traC mutant, differs from its parent R100-1 only in that it contains an additional copy of IS1 inserted within the tra gene region of 82.1 kb. R100-31, atraX, TC-s mutant of R100-1, is deleted in R100-1 sequences starting at one of the IS3 termini (46.9 kb) and extending with RTF to 61.0 kb. Heteroduplex studies of F plasmids with the DNA of a lambda phage bearing insertion sequence IS2 show that the sequence of F with coordinates 16.3-17.6F is IS2. The occurrence of IS1 at the two junctions of R-determinant DNA and RTF DNA in R plasmids provides a structural basis to explain the mechanism of the previously observed formation of molecules containing one RTF unit and several tandem copies of the R-determinant unit, when R plasmids in Proteus mirabilis are grown in the presence of antibiotics, and the segregation of an R plasmid into an RTF unit and an R-determinant unit. In general, correlation of our results with previous studies shows that insertion sequences play a role in a variety of F- and R-related intra- and intermolecular recombination phenomena.     

Construction of a human X-chromosome-enriched phage library which facilitates analysis of specific loci

A human X-chromosome-enriched MboI-partial-digest recombinant library in phage lambda Charon30 has been constructed. Twelve out of the thirteen X-chromosome DNA sequences that were tested were present in the library. Most regions were covered in overlapping phage inserts; mean insert size was 13.7 kb. One phage from the library allowed detection of a 225-bp insertion of DNA into a region near the Duchenne muscular dystrophy (DMD) locus. Another recombinant phage represents an expansion of a region which exhibits extensive and varying homology with other human chromosomes, including the Y, as well as with rodent DNA. The present library should have widespread use for examining DNA sequences on the human X chromosome.     

Splicing of the adenovirus-2 E1A 13S mRNA requires a minimal intron length and specific intron signals.

The adenovirus E1A region encodes three overlapping mRNAs, designated 9S, 12S and 13S. They differ from each other with regard to the length of the intron which is removed by RNA splicing. We have constructed E1A genes with deletions and insertions in the intervening sequence that is common to all three E1A mRNAs, in a search for signals which influence splicing of the 13S mRNA. Mutant plasmids were transfected into HeLa cells and the transiently expressed E1A mRNAs characterized by the S1 protection assay. The results show that five upstream and 20 downstream nucleotides are sufficient to allow for a correct utilization of the 5'-splice junction for the E1A 13S mRNA. Moreover, we show that a minimal intron length of 78 nucleotides is required for efficient 13S mRNA splicing. The ability of mutants with large intron deletions to maturate a 13S mRNA could partially be restored by expanding the intron length with phage lambda sequences. However, in no case was the normal splicing efficiency obtained with these mutants. In contrast, one mutant in which sequences from the authentic 13S mRNA intron were used to expand the intron expressed almost normal levels of 13S mRNA, thus suggesting that signals which specifically promote 13S mRNA splicing exist.     

Plasmid transduction using the D3112DeltaH miniphage in Pseudomonas aeruginosa cells]

Plasmid DNA transduction with mini-D3112 delta H, deletion derivative of phage D3112, which lost the genes essential for phage growth but retained the sites required for transposition and packaging was studied. Unlike D3112, mini-D3112 delta H element can transduce plasmids and plasmid markers at frequencies of 10(-5)-10(-8) in rec+ cells of Pseudomonas aeruginosa. Plasmids R1162 and R388 of the size smaller than phage genome were transduced intact. Large plasmids, like RP4 and R151, were deleted under transduction. By this way, we isolated deletion derivatives of RP4. The smallest derivative pN2 contained a 4.5 kb fragment of RP4. Unlike the latter, pN2 plasmid had narrow host range and did not maintain in Escherichia coli cells.