Vacuum blotting: a simple method for transferring DNA from sequencing gels to nylon membranes

Efficient Agrobacterium tumefaciens-mediated stable genetic transformation of green microalgae, Chlorella sorokiniana

The green oleaginous microalgae, Chlorella sorokiniana, is a highly productive Chlorella species and a potential host for the production of biofuel, nutraceuticals, and recombinant therapeutic proteins. The lack of a stable and efficient genetic transformation system is the major bottleneck in improving this species. We report an efficient and stable Agrobacterium tumefaciens-mediated transformation system for the first time in C. sorokiniana. Cocultivation of C. sorokiniana cells (optical density at λ ₆₈₀ = 1.0) with Agrobacterium at a cell density of OD₆₀₀ = 0.6, on BG11 agar medium (pH 5.6) supplemented with 100 μM of acetosyringone, for three days at 25 ± 2 °C in the dark, resulted in significantly higher transformation efficiency (220 ± 5 hygromycin-resistant colonies per 10⁶ cells). Transformed cells primarily selected on BG11 liquid medium with 30 mg/L hygromycin followed by selecting homogenous transformants on BG11 agar medium with 75 mg/L hygromycin. PCR analysis confirmed the presence of hptII, and the absence of virG amplification ruled out the Agrobacterium contamination in transformed microalgal cells. Southern hybridization confirmed the integration of the hptII gene into the genome of C. sorokiniana. The qRT-PCR and Western blot analyses confirmed hptII and GUS gene expression in the transgenic cell lines. The specific growth rate, biomass doubling time, PSII activity, and fatty-acid profile of transformed cells were found similar to wild-type untransformed cells, clearly indicating the growth and basic metabolic processes not compromised by transgene expression. This protocol can facilitate opportunities for future production of biofuel, carotenoids, nutraceuticals, and therapeutic proteins.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-02750-7.

The conserved 7SK snRNA gene localizes to human chromosome 6 by homolog exclusion probing of somatic cell hybrid RNA

Many small RNAs contribute essential activities to eukaryotic cells. In mammalian genomes dispersed repetitive sequences which exhibit homology to small RNAs often exist as pseudogenes which can complicate identification, localization, and analysis of the authentic gene. We mapped a productive human 7SK small nuclear RNA gene to human chromosome 6 by analyzing Northern blots derived from a panel of somatic cell hybrids that contain single human chromosomes. In order to avoid crossreactivity of the probe with rodent 7SK RNA, which is 98% identical to human 7SK, a method termed homolog exclusion probing was developed. This method uses an excess of non-labelled rodent-specific oligodeoxynucleotide to block the rodent 7SK RNA from hybridizing with the human-specific oligodeoxynucleotide probe. The effectiveness of this method to enhance the human 7SK RNA signal is demonstrated. The potential to map and subsequently isolate other small RNA genes by this approach and the use of homolog exclusion probing to discriminate among family members of highly related RNAs and DNAs in a single species is discussed.

Multiple dispersed loci produce small cytoplasmic Alu RNA

Alu repeats are short interspersed elements (SINEs) of dimeric structure whose transposition sometimes leads to heritable disorders in humans. Human cells contain a poly(A)- small cytoplasmic transcript of -120 nucleotides (nt) homologous to the left Alu monomer. Although its monomeric size indicates that small cytoplasmic Alu (scAlu) RNA is not an intermediary of human Alu transpositions, a less abundant poly(A)-containing Alu transcript of dimeric size and specificity expected of a transposition intermediary is also detectable in HeLa cells (A. G. Matera, U. Hellmann, M. F. Hintz, and C. W. Schmid, Mol. Cell. Biol. 10:5424-5432, 1990). Although its function is unknown, the accumulation of Alu RNA and its ability to interact with a conserved protein suggest a role in cell biology (D.-Y. Chang and R. J. Maraia, J. Biol. Chem. 268:6423-28, 1993). The relationship between the -120- and -300-nt Alu transcripts had not been determined. However, a B1 SINE produces scB1 RNA by posttranscriptional processing, suggesting a similar pathway for scAlu. An Alu SINE which recently transposed into the neurofibromatosis 1 locus was expressed in microinjected frog oocytes. This neurofibromatosis 1 Alu produced a primary transcript followed by the appearance of the scAlu species. 3' processing of a synthetic -300-nt Alu RNA by HeLa nuclear extract in vitro also produced scAlu RNA. Primer extension of scAlu RNA indicates synthesis by RNA polymerase III. HeLa-derived scAlu cDNAs were cloned so as to preserve their 5'-terminal sequences and were found to correspond to polymerase III transcripts of the left monomeric components of three previously identified Alu SINE subfamilies. Rodent x human somatic cell hybrids express Alu RNAs whose size, heterogeneous length, and chromosomal distribution indicate their derivation from SINEs. The coexpression of dimeric and monomeric Alu RNA in several hybrids suggests a precursor-product relationship.

Multiple dispersed loci produce small cytoplasmic Alu RNA

Alu repeats are short interspersed elements (SINEs) of dimeric structure whose transposition sometimes leads to heritable disorders in humans. Human cells contain a poly(A)- small cytoplasmic transcript of -120 nucleotides (nt) homologous to the left Alu monomer. Although its monomeric size indicates that small cytoplasmic Alu (scAlu) RNA is not an intermediary of human Alu transpositions, a less abundant poly(A)-containing Alu transcript of dimeric size and specificity expected of a transposition intermediary is also detectable in HeLa cells (A. G. Matera, U. Hellmann, M. F. Hintz, and C. W. Schmid, Mol. Cell. Biol. 10:5424-5432, 1990). Although its function is unknown, the accumulation of Alu RNA and its ability to interact with a conserved protein suggest a role in cell biology (D.-Y. Chang and R. J. Maraia, J. Biol. Chem. 268:6423-28, 1993). The relationship between the -120- and -300-nt Alu transcripts had not been determined. However, a B1 SINE produces scB1 RNA by posttranscriptional processing, suggesting a similar pathway for scAlu. An Alu SINE which recently transposed into the neurofibromatosis 1 locus was expressed in microinjected frog oocytes. This neurofibromatosis 1 Alu produced a primary transcript followed by the appearance of the scAlu species. 3' processing of a synthetic -300-nt Alu RNA by HeLa nuclear extract in vitro also produced scAlu RNA. Primer extension of scAlu RNA indicates synthesis by RNA polymerase III. HeLa-derived scAlu cDNAs were cloned so as to preserve their 5'-terminal sequences and were found to correspond to polymerase III transcripts of the left monomeric components of three previously identified Alu SINE subfamilies. Rodent x human somatic cell hybrids express Alu RNAs whose size, heterogeneous length, and chromosomal distribution indicate their derivation from SINEs. The coexpression of dimeric and monomeric Alu RNA in several hybrids suggests a precursor-product relationship.

Conditional silencing: the HMRE mating-type silencer exerts a rapidly reversible position effect on the yeast HSP82 heat shock gene

The HMRE silencer of Saccharomyces cerevisiae has been previously shown to transcriptionally repress class II and class III genes integrated within the HMR silent mating-type locus up to 2.6 kb away. Here we study the ability of this element to repress at an ectopic position, independent of sequences normally associated with it. When integrated 750 bp upstream of the HSP82 heat shock gene, the silencer represses basal-level transcription approximately 5-fold but has no effect on chemical- or heat-shock-induced expression. Such conditional silencing is also seen when the HMRE/HSP82 allele is carried on a centromeric episome or when the entire HMRa domain is transplaced 2.7 kb upstream of HSP82. Notably, the a1 promoter within the immigrant HMRa locus remains fully repressed at the same time HSP82 is derepressed. The position effect mediated by the E silencer is absolutely dependent on the presence of a functional SIR4 gene product, is lost within 1 min following stress induction, and is fully reestablished within 15 min following a return to nonstressful conditions. Similar kinetics of reestablishment are seen in HMRE/HSP82 and HMRa/HSP82 strains, indicating that complete repression can be mediated over thousands of base pairs within minutes. DNase I chromatin mapping reveals that the ABF1, RAP1, and autonomously replicating sequence factor binding sites within the silencer are constitutively occupied in chromatin, unaltered by heat shock or the presence of SIR4. Similarly, the heat shock factor binding site upstream of HSP82 remains occupied under such conditions, suggesting concurrent occupancy of silencer and activator binding sites. Our results are consistent with a model in which silencing at the HMRE/HSP82 allele is mediated by direct or indirect contacts between the silencer protein complex and heat shock factor.

Conditional silencing: the HMRE mating-type silencer exerts a rapidly reversible position effect on the yeast HSP82 heat shock gene

The HMRE silencer of Saccharomyces cerevisiae has been previously shown to transcriptionally repress class II and class III genes integrated within the HMR silent mating-type locus up to 2.6 kb away. Here we study the ability of this element to repress at an ectopic position, independent of sequences normally associated with it. When integrated 750 bp upstream of the HSP82 heat shock gene, the silencer represses basal-level transcription approximately 5-fold but has no effect on chemical- or heat-shock-induced expression. Such conditional silencing is also seen when the HMRE/HSP82 allele is carried on a centromeric episome or when the entire HMRa domain is transplaced 2.7 kb upstream of HSP82. Notably, the a1 promoter within the immigrant HMRa locus remains fully repressed at the same time HSP82 is derepressed. The position effect mediated by the E silencer is absolutely dependent on the presence of a functional SIR4 gene product, is lost within 1 min following stress induction, and is fully reestablished within 15 min following a return to nonstressful conditions. Similar kinetics of reestablishment are seen in HMRE/HSP82 and HMRa/HSP82 strains, indicating that complete repression can be mediated over thousands of base pairs within minutes. DNase I chromatin mapping reveals that the ABF1, RAP1, and autonomously replicating sequence factor binding sites within the silencer are constitutively occupied in chromatin, unaltered by heat shock or the presence of SIR4. Similarly, the heat shock factor binding site upstream of HSP82 remains occupied under such conditions, suggesting concurrent occupancy of silencer and activator binding sites. Our results are consistent with a model in which silencing at the HMRE/HSP82 allele is mediated by direct or indirect contacts between the silencer protein complex and heat shock factor.

Electrophoretic transfer of DNA from gels to filters by using a homemade apparatus

Electroblotting is a rapid and high-efficiency method of transferring DNA from gels to the variety of membranes available, in very short time frames when compared with capillary-blotting techniques and with much less preparation required. A simple apparatus made of two food savers, one dialysis membrane, one sponge, and two removable electrodes was used to transfer DNA molecules up to 21 kb, from agarose gels to nitrocellulose filters, in less than 30 min. A recombinant pAT153 plasmid carrying the human cytomegalovirus BamH I-P fragment (7.2 kb) was rapidly identified by hybridization, using our simplified transfer procedure.

The yeast heat shock response is induced by conversion of cells to spheroplasts and by potent transcriptional inhibitors

We report here that procedures commonly used to measure transcription and mRNA decay rates in Saccharomyces cerevisiae induce the heat shock response. First, conversion of cells to spheroplasts with lyticase, a prerequisite for nuclear runoff transcription, induces the expression of HSP70 and HSP90 heat shock genes. The transcript levels of the non-heat-shock gene ACT1 are slightly depressed, consistent with the general yeast stress response. Second, the DNA intercalator, 1,10-phenanthroline, widely employed as a general transcriptional inhibitor in S. cerevisiae, enhances the mRNA abundance of certain heat shock genes (HSP82, SSA1-SSA2) although not of others (HSC82, SSA4, HSP26). Third, the antibiotic thiolutin, previously demonstrated to inhibit all three yeast RNA polymerases both in vivo and in vitro, increases the RNA levels of HSP82 5- to 10-fold, those of SSA4 greater than 25-fold, and those of HSP26 greater than 50-fold under conditions in which transcription of non-heat-shock genes is blocked. By using an episomal HSP82-lacZ fusion gene, we present evidence that lyticase and thiolutin induce heat shock gene expression at the level of transcription, whereas phenanthroline acts at a subsequent step, likely through message stabilization. We conclude that, because of the exquisite sensitivity of the yeast heat shock response, procedures designed to measure the rate of gene transcription or mRNA turnover can themselves impact upon each process.

The subset of mouse B1 (Alu-equivalent) sequences expressed as small processed cytoplasmic transcripts

B1 (Alu-equivalent) is a murine short interspersed element whose amplification probably involved an RNA intermediate. B1-homologous RNA comprise a population of heterogenous transcripts of questionable function. A cloned B1 is expressed in the injected frog oocyte by RNA polymerase III transcription, ribonucleoprotein formation, post-transcriptional 3'-processing, and nucleocytoplasmic transport. The present study characterizes small cytoplasmic B1 transcripts of mouse cells. Analyses of ten cDNA clones revealed a subset of a high degree of sequence identity (98%) from which a novel consensus was developed. Structural analyses of these RNAs demonstrated a conserved Alu domain originally identified as part of the 7SL RNA within the translational control domain of the signal recognition particle, while this structure was not conserved in the majority of B1s in the sequence database. Furthermore, it was demonstrated that 3'-processing occurred in only a subset of B1 transcripts in-vitro using homologous nuclear extracts, and in the injected oocyte. The data demonstrate that a limited set of B1 sequences are expressed as processed RNA polymerase III-transcripts of a high degree of structural conservation. Although this subset is transcriptionally active, the selective expression may be due to regulation at the levels of processing and cytoplasmic accumulation. Their lack of Poly-(A) or 3'-oligo-(U) tracts argue that these RNAs are unlikely to represent transposition intermediates. Rather, their cytosolic compartmentalization and conservation of a biologically recognized structure, suggests potential involvement in other aspects of cellular metabolism.

Genomic footprinting of the yeast HSP82 promoter reveals marked distortion of the DNA helix and constitutive occupancy of heat shock and TATA elements

We describe here for the first time successful application of the hydroxyl radical technique for genomic footprinting. In combination with two complementary techniques, DNase I footprinting and dimethyl sulfate methylation protection, we have obtained a high-resolution map of the promoter region of the yeast HSP82 heat shock gene, which resides within a constitutive nuclease hypersensitive site. We find that irrespective of transcriptional state, basal or induced, only one of three putative heat shock elements, HSE1, and the TATA box are tightly bound by proteins, presumably heat shock factor (HSF) and TFIID, respectively. Whereas the HSE1-associated factor binds tightly within the major groove of DNA, as discerned by protection of guanine residues from methylation by dimethyl sulfate in intact cells, the TATA factor appears to bind principally to the sugar-phosphate backbone, as revealed by strong protection from hydroxyl radical cleavage in whole-cell lysates. In addition, while HSE1 is strongly footprinted by DNase I in lysates, the TATA box is only weakly footprinted. Strikingly, both elements are associated with marked distortion of the DNA double helix in chromatin. Protein binding to HSE1 appears to cause a non-B-conformation, on the basis of a local 12 base-pair periodicity of hydroxyl radical protection and the presence of multiple DNase I hyperreactive cleavages flanking HSE1, whose pattern changes following heat shock. Similarly, helix distortion is evident in the vicinity of the TATA box, since hydroxyl radical detects a lower strand-specific hypersensitive site at the dyad center of an adjacent polypurine tract. Finally, the absence of discernable modulation in the DNase I cleavage pattern argues against the presence of a specifically positioned nucleosome within the IISP82 promoter region.

Promoter function and in situ protein/DNA interactions upstream of the yeast HSP90 heat shock genes

We have mapped in vivo protein/DNA interactions within the upstream regulatory regions of the two yeast HSP90 genes, and have begun mutagenizing footprinted sequences in an effort to identify the cis-acting determinants of heat shock transcription. Genomic footprinting of the HSP82 promotor using chemical and enzymatic nucleases reveals that irrespective of transcriptional state, the most proximal of three heat shock elements, HSE1, is occupied along both sugar-phosphate backbones as well as within its major groove, while the TATA box is bound along both sugar-phosphate backbones. Distorted DNA structure is associated with each constitutively bound factor: protein binding to HSE1 appears to induce a local A-form-like helical conformation, whereas occupancy of the TATA box is associated with strand-specific nuclease hypersensitivity of an adjacent polypurine tract. In situ mutagenesis experiments indicate that HSE1 is absolutely required for both basal and induced expression, and that basal transcription can be preferentially abolished by point mutations within this sequence. In contrast, point mutations within the TATA element have the reverse effect, as induced transcription is more significantly affected. Similar to HSE1 point mutants, we have found that basal transcription is preferentially repressed by an HMRE silencer element when it is transplaced approximately 1 kb upstream of the HSP82 start site. Finally, a complementary footprinting analysis of the upstream region of the constitutively expressed HSC82 gene reveals the presence of three discrete protein complexes. These map to the TATA box, the promotor-distal heat shock element, C.HSE1, and a novel sequence upstream of C. HSE1, suggesting that the 10-fold higher basal transcription of HSC82 stems, at least in part, from a non-HSE-binding factor.

Alkaline transfer of small restriction fragments from polyacrylamide gels

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Basal-level expression of the yeast HSP82 gene requires a heat shock regulatory element

Previous studies have shown that heat shock factor is constitutively bound to heat shock elements in Saccharomyces cerevisiae. We demonstrate that mutation of the heat shock element closest to the TATA box of the yeast HSP82 promoter abolishes basal-level transcription without markedly affecting inducibility. The mutated heat shock element no longer bound putative heat shock factor, either in vitro or in vivo, but still resided within a nuclease-hypersensitive site in the chromatin. Thus, constitutive binding of heat shock factor to heat shock elements in S. cerevisiae appears to functionally direct basal-level transcription.

Basal-level expression of the yeast HSP82 gene requires a heat shock regulatory element

Previous studies have shown that heat shock factor is constitutively bound to heat shock elements in Saccharomyces cerevisiae. We demonstrate that mutation of the heat shock element closest to the TATA box of the yeast HSP82 promoter abolishes basal-level transcription without markedly affecting inducibility. The mutated heat shock element no longer bound putative heat shock factor, either in vitro or in vivo, but still resided within a nuclease-hypersensitive site in the chromatin. Thus, constitutive binding of heat shock factor to heat shock elements in S. cerevisiae appears to functionally direct basal-level transcription.