Escherichia coli RNA-polymerase binding sites on DNA are only 14 base pairs long and are located between sequences that are very rich in AplusT

Interaction of nucleic acids with the DNA-dependent RNA polymerases of Drosophila

The interaction between the three Drosophila DNA-dependent RNA polymerases (EC 2.7.7.6) and the DNA template or the RNA product was investigated by photochemical cross-linking and binding studies, using RNA polymerase subunits immobilized on nitro-cellulose filters. It can be shown that the two largest subunits are responsible for the binding of the enzymes to both template and newly-synthesized RNA.

DNA strand specificity in promoter recognition by RNA polymerase

DNA strand and enzyme subunit specificities involved in the interaction between E. coli RNA polymerase and T7 DNA were studied by photo-crosslinking techniques. In non-specific enzyme-DNA complexes, subunits, sigma, beta, and beta' were crosslinked to both strands of the DNA. Under conditions leading to specific enzyme-promoter complexes, however, only sigma and beta subunits were crosslinked. The sigma subunit was crosslinked preferentially to the non-sense strand at promoter sites. No such strand specificity was observed for the beta subunit. These results provide insight into the molecular mechanism of promoter recognition and indicate that the interaction between RNA polymerase and DNA template is different at promoters and at non-specific sites.

Transcription activity of a DNA-protein complex isolated from spinach plastids

A DNA . protein complex of about 150 S is isolated from purified spinach chloroplasts by Sepharose 4B gel filtration. A DNA-dependent RNA polymerase activity is found associated with the complex. This DNA protein complex is able to initiate RNA chains in vitro. The RNA synthesis is more dependent on CTP than other nucleoside triphosphates. 50% of the activity is still present with 0.6 M KCl. The temperature optimum occurs between 30 degrees C and 35 degrees C. Rifampicin and rifamycin SV have no inhibitory effect. TNA products have been characterized by gel filtration and by hybridization with chloroplast DNA (ctDNA). At the beginning of transcription DNA products are linked to the transcription complex and are later detached. The molecular weight of the product ranges between 0.07 X 10(6) and 2 X 10(6). A part of the product (3--4%) has a molecular weight higher than 2 X 10(6). No endogenous RNase activity was present during the molecular weight determinations experiments. Hybridization experiments show that at least 75% of the RNA products are hybridizable with ctDNA and that 40% of these products are composed of chloroplast ribosomal RNA, showing that rDNA is preferentially transcribed.

A study of unwinding of DNA and shielding of the DNA grooves by RNA polymerase by using methylation with dimethylsulphate

The dimethylsulphate method has been used to study the complexes of RNA polymerase (Escherichia coli) with DNA of T7 phage, poly[d(A--T)] and fragments of calf thymus DNA protected against DNase digestion by RNA polymerase. The binding of RNA polymerase to DNA significantly increases the formation of 1-methyl-adenine produced by methylation of the single-stranded DNA region, diminishes by about 10% the formation of 3-methyl-adenine by methylation within the minor groove and does not affect the formation of 7-methyl-guanine by methylation within the major DNA groove. The presence of nascent RNA decreases the formation of 1-methyl-adenine in DNA of the complex by about 30%. The initiation of RNA synthesis or RNA synthesis itself does not influence the methylation of the major groove but shielding of the minor groove increases by about twice as much. These results suggest that RNA polymerase, upon binding, breaks Watson-Crick base-pairing in a DNA region of about 15-base-pairs long, that nascent RNA forms a duplex with DNA of about 10-base-pairs long; and that the enzyme weakly interacts with DNA along its grooves and preferentially makes contacts with the minor groove.

Electron microscopy analysis of the interaction between Escherichia coli DNA-dependent RNA polymerase and the replicative form of phage fd DNA. 1. Mapping of the binding sites

The interaction of Escherichia coli DNA-dependent RNA polymerase (EC 2.7.7.6) with the replicative form of the DNA from the filamentous coliphage fd cleaved by the restriction endonuclease HindII has been studied by electron microscopy at low and high ionic strength. In the presence of ATP or GTP, and heparin, RNA polymerase binds to fd replicative-form DNA at a few specific sites which have been mapped. The map was oriented so that transcription is from right to left. Three main GTP initiator sites are found at 15%, 82% and 94% of the genome length. One main ATP initiator site is found which cannot be mapped with the same accuracy, and which is localized between 38% and 50%. In the absence of initiator triphosphates and heparin, the binding of the enzyme to fd DNA is much more heterogeneous and therefore the mapping is more difficult. Nevertheless it seems that the preferential binding regions correspond to the specific sites mapped in the presence of GTP or ATP. The mean number of polymerase molecules bound to DNA as a function of the molecular ratio enzyme to DNA present in the mixture has been determined. From these results a binding isotherm can be obtained. The apparent equilibrium constant (K approximately 10(9) M-1) which is derived certainly represents an under-estimated value, as discussed.

Influence of DNA acidification on DNA premelting and template properties

Acidification of a T7 DNA sample was found to be partly irreversible as ultraviolet difference spectra measured at various sub-melting temperatures were different from those observed for a 'normal' DNA sample. This implies some subtle conformational change which is not reversed by return to neutral pH. In the same conditions, only poly(purine)-poly(pyrimidine) polymers behaved in a different manner, during premelting, according to whether they were previously acidified or not. The properties of acidified and reneutralized T7 DNA were also investigated for Escherichia coli RNA polymerase binding and transcription. An inhibition of RNA synthesis and chain initiation was observed. The results suggest that the binding of the enzyme is affected. RNA synthesized is specific but there is a decrease in the number and in the stability of the RNA-polymerase-DNA complexes.

Interaction of RNA polymerase from Escherichia coli with DNA. Analysis of T7 DNA early-promoter sites

A method was devised for directing RNA polymerase on a single promoter site on T7 DNA. Initiation complexes were formed on each of the three main promoter sites using one dinucleotide plus one nucleoside triphosphate. The ternary initiation complexes are resistant to rifampicin action, to inhibition by (rI)n at 0 degrees C and are stable at high salt concentrations. A minimum of a trinucleotide is required to form a stable ternary complex. To determine which promoter site was selected by RNA polymerase during initiation, the (rI)n-resistant RNA was digested by RNAse III to generate three characteristic initiator RNA fragments, resolved by gel electrophoresis. The three major promoter sites could be selected individually by using different primer and substrate combinations ApC plus ATP selected promoter A3, CpG plus CTP selected A2 and CpC plus ATP specified preferentially A1. A number of primer-substrate combinations specified each site at low salt concentration but the substrate requirement became very stringent at high salt concentration, suggesting that the postulated local opening of the promoter site could be more or less extensive, depending on the ionic strength. The minimum opening observed at high salt concentration corresponded to the insertion of a leader trinucleotide sequence. The promoter region melted by RNA polymerase at low salt concentration was (G plus C)-rich and corresponded to about 9 to 11 base pairs. Sequences of the melting recognition regions were tentatively inferred from the results.

The effect of modification of T7 DNA by the carcinogen N-1-acetylaminofluorene: termination of transcription in vitro

To study the effects of N-2-acetylaminofluorene (AAF) modification of DNA on transcription, purified DNA from bacteriophage T7 was modified in vitro to varying extent with AAF and transcribed by DNA-dependent RNA polymerase from Escherichia coli. The main effects of AAF modification on transcription are a marked inhibition of the rate and extent of trna synthesis with relatively little effect on initiation except at very high AAF doses. Calibration of the percent modification with [14-C]AAF and analysis of the size of the RNA product by double isotope labeling and polyacrylamide gel electrophoresis support the following mechanism of transcription inhibition: most of the AAF residues bound to the coding strand of the DNA cause premature termination of transcription, at or near the site of modification, with release of RNA polymerase. This results in the production of shorter RNA chains with increasing amounts of bound carcinogen. The data are consistent with there being no reinitiation and/or synthesis of RNA distal to the AAF-modification site.