Evidence that mutations in the suA polarity suppressing gene directly affect termination factor rho

Transcription-Translation Coupling in Bacteria

In bacteria, transcription and translation take place in the same cellular compartment. Therefore, a messenger RNA can be translated as it is being transcribed, a process known as transcription-translation coupling. This process was already recognized at the dawn of molecular biology, yet the interplay between the two key players, the RNA polymerase and ribosome, remains elusive. Genetic data indicate that an RNA sequence can be translated shortly after it has been transcribed. The closer both processes are in time, the less accessible the RNA sequence is between the RNA polymerase and ribosome. This temporal coupling has important consequences for gene regulation. Biochemical and structural studies have detailed several complexes between the RNA polymerase and ribosome. The in vivo relevance of this physical coupling has not been formally demonstrated. We discuss how both temporal and physical coupling may mesh to produce the phenomenon we know as transcription-translation coupling.

Chemical-genetic interrogation of RNA polymerase mutants reveals structure-function relationships and physiological tradeoffs

The multi-subunit bacterial RNA polymerase (RNAP) and its associated regulators carry out transcription and integrate myriad regulatory signals. Numerous studies have interrogated RNAP mechanism, and RNAP mutations drive Escherichia coli adaptation to many health- and industry-relevant environments, yet a paucity of systematic analyses hampers our understanding of the fitness trade-offs from altering RNAP function. Here, we conduct a chemical-genetic analysis of a library of RNAP mutants. We discover phenotypes for non-essential insertions, show that clustering mutant phenotypes increases their predictive power for drawing functional inferences, and demonstrate that some RNA polymerase mutants both decrease average cell length and prevent killing by cell-wall targeting antibiotics. Our findings demonstrate that RNAP chemical-genetic interactions provide a general platform for interrogating structure-function relationships in vivo and for identifying physiological trade-offs of mutations, including those relevant for disease and biotechnology. This strategy should have broad utility for illuminating the role of other important protein complexes.

A Fluorogenic Assay To Monitor Rho-Dependent Termination of Transcription

Transcription termination mediated by the ring-shaped, ATP-dependent Rho motor is a multipurpose regulatory mechanism specific to bacteria and constitutes an interesting target for the development of new antibiotics. Although Rho-dependent termination can punctuate gene expression or contribute to the protection of the genome at hundreds of sites within a given bacterium, its exact perimeter and site- or species-specific features remain insufficiently characterized. New advanced approaches are required to explore thoroughly the diversity of Rho-dependent terminators and the complexity of associated mechanisms. Current in vitro analyses of Rho-dependent termination rely on radiolabeling, gel electrophoresis, and phosphorimaging of transcription reaction products and are thus hazardous, inconvenient, and low-throughput. To address these limitations, we have developed the first in vitro assay using a fluorescence detection modality to study Rho-dependent transcription termination. This powerful experimental tool accurately estimates terminator strengths in a matter of minutes and is optimized for a microplate reader format allowing multiplexed characterization of putative terminator sequences and mechanisms or high-throughput screening of new drugs targeting Rho-dependent termination.

Host factor titration by chromosomal R-loops as a mechanism for runaway plasmid replication in transcription termination-defective mutants of Escherichia coli

Two Escherichia coli genes, rnhA and recG, encode products that disrupt R-loops by hydrolysis and unwinding, respectively. It is known that the propensity for R-loop formation in vivo is increased during growth at 21 degrees C. We have identified several links between rnhA, recG, and R-loop-dependent plasmid replication on the one hand, and genes rho and nusG involved in factor-dependent transcription termination on the other. A novel nusG-G146D mutation phenocopied a rho-A243E mutation in conferring global deficiency in transcription termination, and both mutants were killed at 21 degrees C following overexpression of rnhA(+). Mutant combinations rnhA-nusG or recG-rho were synthetically lethal at 21 degrees C, with the former being suppressed by recG(+) overexpression. rho and nusG mutants were killed following transformation with plasmids such as pACYC184 or pUC19 (which have R-loop replication intermediates) even at 30 degrees C or 37 degrees C, and the lethality was correlated with greatly increased content of supercoiled monomer species of these and other co-resident R-loop-dependent plasmids. Plasmid-mediated lethality in the mutants was suppressed by overexpression of rnhA(+) or recG(+). Two additional categories of trans-acting suppressors of the plasmid-mediated lethality were identified whose primary effects were, respectively, a reduction in plasmid copy number even in the wild-type strain, and a restoration of the proficiency of in vivo transcription termination in the nusG and rho mutant strains. The former category of suppressors included rom(+), and mutations in rpoB(Q513L), pcnB, and polA, whereas the latter included a mutation in rho (R221C) and several non-null mutations (E74K, L26P, and delta64-137) in the gene encoding the nucleoid protein H-NS. We propose that an increased occurrence of chromosomal R-loops in the rho and nusG mutants leads to titration of a cyloplasmic host factor(s) that negatively modulates the stability of plasmid R-loop replication intermediates and consequently to runaway plasmid replication.

Autogenous regulation of the gene for transcription termination factor rho in Escherichia coli: localization and function of its attenuators

We present evidence that the expression of rho is regulated by rho-dependent attenuation of transcription. Gene fusion analysis with nested series of deletions of rho indicated that the transcription of rho is attenuated in a rho-dependent manner in the leader region and that neither a read-through transcription from the upstream gene, trxA, nor a modulation of transcription initiation of the rho promoter is involved in the self-control of rho. S1 mapping and Northern hybridization analyses localized at least six transcription attenuation or termination sites in the region ranging from the 3' end of the trxA structural gene to the middle of the rho structural gene. Among them, the most upstream site overlapping the rho promoter sequence was assigned to the terminator for the trxA gene, and the second and third sites, mapping about 80 and 50 nucleotides upstream from the start codon of rho, were suggested to function as the major attenuation sites for regulation of the rho expression. Further, the start points of the trxA and rho RNAs were determined in an in vitro transcription system to be located 111 nucleotides (U) and 255 nucleotides (G) upstream from their respective start codons. These results necessitate revisions of previous predictions on the sites of transcriptional signals in the trxA and rho genes (S. Brown, B. Albrechtsen, S. Pedersen, and P. Klemm, J. Mol. Biol. 162:283-298, 1982; C.-J. Lim, D. Geraghty, and J. A. Fuchs, J. Bacteriol. 163:311-316, 1985; B.J. Wallace and S.R. Kushner, Gene 32:399-408, 1984).

Autogenous regulation of transcription termination factor Rho

We present evidence that the transcription termination factor Rho is autogenously regulated in Escherichia coli. The steady-state level of Rho is increased approximately tenfold in rho mutant cells. In the rho+ revertants, the content of Rho is similar to the wild-type level. A rho-/rho+ merodiploid produces equimolar amounts of the mutant and the wild-type Rho polypeptides, both at a reduced level compared to the mutant. The steady-state level of rho messenger RNA is also increased in a rho mutant. A rho-galK transcriptional fusion produces at least tenfold more galactokinase in a rho- strain than in a rho+ strain. In vitro, in a coupled transcription-translation system, the synthesis of Rho protein is specifically inhibited by wild-type Rho but not by Rho15 mutant protein. Anti-Rho antibody specifically stimulates Rho synthesis in the rho+ extract but not in a rho- extract. We suggest that the autogenous regulation of Rho involves premature transcription termination within the rho gene. Regulation of Rho level may provide the cell a mechanism to modulate the expression of genes which are separated from their promoters by Rho-dependent termination signals.

Antitermination of transcription from an Escherichia coli ribosomal RNA promoter

The Escherichia coli lac and ara promoters and rrnC ribosomal RNA promoter-leader region were fused to lacZYA. Transcription termination signals were introduced into the lac genes of these fusions by Tn9 and IS1 insertions. Measurement of lac enzymes from upstream and downstream of the insertions showed that termination signals resulting from these insertions are very efficient when transcription begins at lac or ara promoters but are very inefficient when transcription begins at the rrnC promoter-leader region. The rrnC promoter-leader region must, therefore, modify RNA polymerase to enable it to read through transcription termination signals.

Isolation and characterization of rho mutants of Escherichia coli with increased transcription termination activities

A novel type of rho mutants, rhos, with increased transcription termination activities have been isolated. A termination defective rho mutation rho-ts702 (formerly designated nitA702), which causes temperature-sensitive cell growth, was found to be dominant over the wild-type allele in relieving mutational polarity. The rhos mutations were derived as temperature-resistant revertants of rho-ts702 carried by lambda transducing phage. They exhibited dominance over rho-ts702 leading to restoration of polarity. When the rhos mutations were introduced into the Escherichia coli chromosome, they caused increased polarity in the trp and lac operons. The rhos mutants were classified into two groups in terms of their terminator specificity: The first group demonstrated increased termination efficiencies against all terminators tested, whereas the second exhibited various efficiencies, either more than or less than the normal level depending on the terminator. The cellular content of p protein in each rhos strain was significantly lower than that in the rho+ strain. Moreover, in an in vitro transcription system, purified ps proteins showed increased termination activities against the trpE pseudoterminators. These results indicate that the rhos phenotype is due to qualitative alterations, rather than quantitative increases, of the p protein. The reduced content of ps enforces the current notion that the rho gene is autogenously regulated by rho-dependent transcriptional attenuation.

Overproduction of transcription termination factor Rho in Escherichia coli

A plasmid system has been constructed which allows high-level expression of the rho gene of Escherichia coli under the control of the pL promoter and the N-antitermination regulatory system of bacteriophage lambda. The pL-directed synthesis of Rho crucially depends on the lambda N gene product and is promoted most effectively when this product is supplied from the N gene cloned on a separate compatible plasmid with a moderate copy number. The requirement for N can be circumvented partly, but not completely, by deletion of the region preceding the rho structural gene. Attempts were also made to optimize the construction of rho-expression plasmids by adjusting the orientation and location of pL and rho inserts on the pBR322 vector. With optimal conditions, Rho protein is overexpressed 100-fold and can become as much as 10% of the total cellular protein. Using this plasmid system, Rho can be purified with a yield of more than 20 mg from 10 g of induced cells.

Autoregulation of the rho gene of Escherichia coli K-12

It has previously been proposed, based on indirect evidence, that the Rho protein may control the expression of the rho gene. Using an in vitro system for the transcription and translation of the rho gene cloned into plasmid pBR322, we tested this hypothesis directly by monitoring the effect in vitro of excess or limiting Rho protein. The addition of purified Rho protein suppresses Rho synthesis in vitro. The addition of antibody to Rho specifically stimulates Rho synthesis in vitro. The stimulation of Rho factor synthesis by antibody to Rho is reversed by Rho protein. Rho factor purified from a strain with a mutationally altered rho gene (rho-115) does not suppress Rho synthesis in vitro. These results provide convincing evidence that the rho gene is subject to autoregulation.

Temperature-sensitive mutant rho-115 rho-RNA binary complexes, and stabilization by substrates and analogues

To determine the molecular basis for the temperature-sensitivity of pure rho RNA-dependent ATPase from Escherichia coli mutant rho-115 cells, we investigated mutant rho binding to [3H] polyC as measured by retention on nitrocellulose filters. Complexes of wild-type rho and polyC incubated at 37 degrees C and 45 degrees C were similarly stable. At 37 degrees C mutant rho-polyC binary complexes were inactivated at a slightly faster rate than complexes with wild-type rho. Upon shift to 45 degrees C the quantity of rho-115 bound to polyC declined immediately, resulting in one-fifth of the quantity of complexes observed at 37 degrees C. Shift back to 37 degrees C restored the level of observed complexes by two-fold. The inclusion of ATP or the analogue beta-gamma methylene ATP during 45 degrees C incubation resulted in stable mutant rho-polyC complexes. The hydrolysis product ADP was also effective in stabilizing binary complexes at 45 degrees C but this effect was observed with an order of magnitude more ADP than ATP. Adenine, adenosine, AMP or Pi had no stabilizing effect. We conclude that the mutant rho-115 protein exhibits a structural instability as a result of binding RNA. Furthermore ATP confers a wild-type phenotype upon rho-115 protein, probably as a result of conformational change due to binding of this compound. The effect of ATP on the stability of mutant rho-polyC binary complexes supports the model of ATP modulation of rho-RNA interaction proposed by Galluppi and Richardson (1980).

Relative map location of the rep and rho genes of Escherichia coli

The rep gene of Escherichia coli was mapped between ilvC and rho by three-factor P1 transductional crosses and also by complementation with a set of lambda transducing phages that contain known amounts of bacterial DNA linked to ilvC. The physical distance between ilvC and rep and between rep and rho were calculated with an accuracy of +/- 0.4 kilobase to be 0 less than or equal to ilvC-rep less than or equal to 3.4 kilobases and 2.0 less than or equal to rep-rho less than or equal to 6.0 kilobases. It was shown that rho-15 is Gro+ for phage ST-1. An ilv::Tn10 mutation was located in ilvY.

Procedure for purification of Escherichia coli ribonucleic acid synthesis termination protein rho

An improved purification procedure is described for the rho transcription termination factor of Escherichia coli. The method involves lysozyme--sodium deoxycholate lysis, Polymin P fractionation, and chromatography on phosphocellulose, poly(uridylic acid)--Sepharose, and AMP--agarose. The method yields up to 9 mg of electrophoretically pure protein from 200 200 g of E. coli MRE 600. From quantitative amino acid analysis rho is calculated to have an E280nm (1%) of 3.7 +/- 0.3. The purified rho has an ATPase specific activity of 32 nmol of Pi released min-1 microgram-1 when poly(cytidylic acid) is used as a cofactor, and it functions effectively in termination of T7 DNA transcription. A subunit molecular weight of 48 000 for rho was determined by phosphate-buffered sodium dodecyl sulfate--polyacrylamide gel electrophoresis. The amino acid composition and circular dichroism spectrum in the far-ultraviolet for rho are presented.

Relation between UV suppression of polarity in phi X174 and UV sensitivity of rho mutants

The suppression of polarity by UV irradiation was similar to the suppression by rho mutants. This was demonstrated for a polar nonsense mutant of phage phi X174. Treatment of the host for 30 min with 100 micrograms of the radiomimetic drug mitomycin C per ml was about as effective as 550 J of UV irradiation per m2 in relieving polarity. The shape of the UV survival curves for rho mutants could be linked to a proposed mechanism of UV relief of polarity. Host cell reactivation of phage lambda and W-reactivation of phage G4 were unaffected by rho mutations. UV suppression of polarity is independent of the Hcr- and RecA- phenotypes. An explanation for the UV sensitivity of rho mutants is provided, and several ways are considered in which UV irradiation may deplete cellular rho activity and thereby cause UV relief of polarity. We propose a novel theory that relates the UV inactivation of normal repair-proficient cells to a decrease in rho activity.

A mutant rho ATPase from Escherichia coli that is temperature-sensitive in the presence of RNA

The Escherichia coli mutant rho-115 suppresses lac operon polarity conferred by the lacZ::IS1 insertion MS319. The ATPase activity of purified rho-115 protein was maximal at 40 degrees C, in contrast to 45 degrees C for rho+. At higher temperatures (50 degrees C, 55 degrees C), the fractions of activities at maximal temperature were consistently lower for rho-115 compared to rho+. The 30-minute time course of rho-115 ATP hydrolysis was linear at 37 degrees C but at 45 degrees C the linear kinetics of hydrolysis reached a plateau between 10 and 15 minutes. The 30-minute time courses for rho+ were linear at both 37 degrees C and 45 degrees C. The rho-115 and rho+ ATPase activities were equally heat-stable during preincubation at 45 degrees C in buffer. Inclusion of ATP during preincubation protected these rho proteins from inactivation to the same extent. The presence of polyC during preincubation protected rho+ activity but produced substantial inactivation of rho-115 ATPase. The presence of polyU during preincubation gave similar results. Concentrations of polyC between 625 ng/ml and 100 micrograms/ml yielded the same extent of rho-115 ATPase inactivation during preincubation at 45 degrees C. Thermal inactivation of rho-115 ATPase by polyC was halted by shifting preincubation temperature from 45 degrees C to 35 degrees C, indicating that polyC-induced destabilization of rho-115 was irreversible.

Relief of polarity caused by transposon Tn5: application in mapping a cloned region of the Escherichia coli uvrB locus essential for UV resistance

The structure and function of recombinant plasmid pNP5, which consists of vector pMB9 and a 2.5 kb EcoRI fragment harbouring the Escherichia coli uvrB gene, has been investigated. Insertional inactivation with the transposons Tn1 (Apr) or Tn5 (Kmr) has been used to determine the region on pNP5 DNA that is essential for UV resistance in uvrB deletion strains. This region spans approx. 1.8 kb and is separated by at least 280 bp from the pMB9 promoter to which it has been fused. Furthermore, a procedure is described to eliminate the polarity exerted by the transposon Tn5. A combination of in vitro digestion of pNP5::Tn5 DNA with restriction endonuclease XHoI, followed by ligation and subsequent in vivo propagation of the resulting plasmid DNA yields predominantly pNP5 molecules with a site-specific nonpolar mutation. The method allows an investigation of cloned complex genetic units, such as operons.

The ral gene of phage lambda. I. Identification of a non-essential gene that modulates restriction and modification in E. coli

Host controlled restriction in Escherichia coli can be relieved by pre-infecting restricting cells with modified lambda helper phages. This process, in which intact unmodified phage genomes are allowed to escape restriction attack, is mediated by a newly identified lambda function called ral. The ral gene has been located by deletion mapping between cIII and N. Efficient expression of the ral gene requires the product of the regulator gene N. Polyacrylamide gel analysis of the lambda proteins specified by the cIII-N region failed to reveal the product of the ral gene, but demonstrated that protein Ea10 is encoded by a gene located immediately to the left of ral. From these results the map order cIII-Ea10-ral-TL1-N was deduced. Ral specifically alleviates restriction in E. coli K and E. coli B, but does not affect restriction systems EcoRI, EcoRII and EcoP1. In addition, ral enhances the modification activity of the EcoK and EcoB restriction enzymes: we observed that efficient modification of progeny phages obtained by propagating unmodified lambda phages in r-m+ hosts, is dependent upon the presence of ral. We thus conclude that the ral gene product acts by modulating the restriction and modification activities of the type I restriction systems in E. coli, and the possible mechanisms will be discussed.

Control of early gene expression of bacteriophage T4: involvement of the host rho factor and the mot gene of the bacteriophage

Many early mRNA species of bacteriophage T4 are not synthesized after infection of Escherichia coli in the presence of chloramphenicol. This has been interpreted as a need for T4 protein(s) to be synthesized to allow expression of some early genes, e.g., those for deoxycytidinetriphosphatase, deoxynucleosidemonophosphate kinase and UDP-glucose-DNA beta-glucosyltransferase. In the experiments described here, early mRNA of bacteriophage T4 was allowed to accumulate during chloramphenicol treatment. After the addition of rifampin to inhibit further RNA synthesis, and subsequent removal of chloramphenicol, the accumulated mRNA was permitted to express itself into measured enzyme activities. It was shown that the early mRNA species coding for deoxycytidinetriphosphatase and UDP-glucose-DNA beta-glucosyltransferase could be formed in the presence of chloramphenicol if the E. coli host cell carried a mutation in the structural gene for the RNA chain termination factor rho. This was interpreted to mean that T4 protein(s) with anti-rho activity is normally required for the expression of these two early genes. An altered rho-factor could not, however, relieve the need of phage protein synthesis for the formation of another early mRNA, that coding for deoxynucleosidemonophosphate kinase. In this case the mot gene of T4 seemed to be involved, since the primary infection of E. coli cells with the mot gene mutant tsG1 did not allow subsequent deoxynucleoside monophosphate kinase mRNA synthesis after wild-type phage infection in the presence of chloramphenicol. In control experiments, deoxynucleoside monophosphate kinase mRNA synthesis induced by wild-type phage superinfecting in the presence of chloramphenicol was facilitated by the primary infection with T4 phage containing an unmutated mot gene.

Transcription and membrane attachment of bacteriophage lambda DNA in the absence of N function in the E. coli suA 1 mutant

In the polarity suppressor strain psuA 1, we observe a partial N independence of both transcription and DNA-membrane attachment for a lambda NN mutant. These results, in agreement with the genetical data reported by Dambly et al. (1976), suggest that the N product and rho factor are involved in the same process but may not interact directly.

Expression of the cloned uvrB gene of Escherichia coli: dependency on nonsense suppressors

Recombinant plasmid pNP5, consisting of plasmid pMB9 on which the uvrB gene is cloned, fully complements for the defects due to chromosomal uvrB mutations in the presence of the amber suppressor sup-6 or supF. Correndonuclease II activity was also completely restored in in UvrB strains containing both plasmid pNP5 and amber suppressor sup-6, as compared with the parental UvrB+ strain. It is shown that the amber mutation which interferes with the expression of the cloned uvrB gene is located outside this gene. Apparently, the amber mutation exerts a polar effect on uvrB expression that is relieved by sup-6 or supF. Introduction of a rho mutation into suppressor-free UvrB strains, harboring pNP5, did not relieve the polarity caused by the amber mutation.

Suppression of polar effects of nonsense mutations by ultraviolet irradiation

Nonsense mutations in capsid genes F and G of phages S13 and phi X174 decreased the expression of genes downstream. These polar effects were suppressed by ultraviolet irradiation of the host before infection. Activities of the downstream genes were restored to between 30 and 95% of their normal levels, as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the viral proteins. The effect of ultraviolet irradiation may be to suppress rhodependent termination of transcription. The polarity of one mutation was poorly suppressed by ultraviolet irradiation.

Regulation of amino acid transport in Escherichia coli by transcription termination factor rho

Amino acid transport rates and amino acid binding proteins were examined in a strain containing the rho-120 mutation (formerly SuA), which has been shown to lower the rho-dependent, ribonucleic acid-activated adenosine triphosphatase activity to 9% of the rho activity in the isogenic wild-type strain. Tryptophan and proline transport, which occur by membrane-bound systems, were not altered. On the other hand, arginine, histidine, leucine, isoleucine, and valine transport were variably increased by a factor of 1.4 to 5.0. Kinetics of leucine transport showed that the LIV (leucine, isoleucine, and valine)-I (binding protein-associated) transport system is increased 8.5-fold, whereas the LIV-II (membrane-bound) system is increased 1.5-fold in the rho mutant under leucine-limited growth conditions. The leucine binding protein is increased fourfold under the same growth conditions. The difference in leucine transport in these strains was greatest during leucine-limited growth; growth on complex media repressed both strains to the same transport activity. We propose that rho-dependent transcriptional termination is important for leucine-specific repression of branched-chain amino acid transport, although rho-independent regulation, presumably by a corepressor-aporepressor-type mechanism, must also occur.

Bacteria with defective rho factors suppress the effects of N mutations in bacteriophage lambda

A prediction based on the model for N-gene function of bacteriophage lambda proposed by Roberts (1971) is confirmed by showing that a lambdaN- double mutant is able to grow in strains of E. coli with defective rho transcription termination factors. The burst sizes for lambdaN- in these strains range from 5 to 24% the burst sizes for lambdaN+ in the same strain. This low level of suppression is also evident in the levels of synthesis of the lambda exonuclease and is consistent with other evidence that the defect in the rho factors of these strains is only partial. These strains do not suppress the effects of mutations in genes O, P and Q of lambda nor in genes 30 and 43 of bacteriophage T4. The lack of suppression of lambdaQ- is significant because the Q-gene product, like the N-gene product, is believed to function as an anti-terminator of lambda transcription but at termination sites that may not require rho factor action.

Suppression of polarity of insertion mutations within the gal operon of E. coli

Phenotypic revertants of galOP::IS1 and galOP::IS2 mutations have been isolated after mutagenesis with nitrosoguanidine, they are probably caused by mutations in gene suA. The polarity suppressor mutations described in this study and a known mutation in gene suA isolated by D. Morse (Morse and Guertin, 1972) suppress polarity caused by IS1 more effectively than that caused by IS2 or IS4. Furthermore, suppressibility is influenced by the site and orientation of IS integration. The synthesis of the three enzymes in galOP::IS suA double mutants is constitutive and the ratio of the three enzymes is altered in comparison to the wild type. The reasons for constitutive synthesis of the galactose enzymes and for the altered ratio of enzyme synthesis are discussed.

Escape synthesis of RNA polymerase subunits and termination factor rho following induction of prophage lambda in Escherichia coli

Synthesis of RNA polymerase subunits and of transcription termination factor p was studied after thermoinduction of prophage lambdac1857 located at several unusual sites on the chromosome of Escherichia coli. When a lysogen carrying the prophage at the bfe gene was induced at 42 degrees C, the rate of synthesis of core polymerase subunits (alpha, beta and beta') rapidly decreased, followed by a marked increase after about 10 min. The latter increase was observed specifically in the "bfe lysogen" and not in any of the other lysogens tested. Similarly, the rate of synthesis of p factor increased appreciably in the induced ilv lysogen carrying the prophage at the ilv gene, and possibly in the bfe lysogen as well, but not in other lysogens examined. Taken together with other evidence, these results suggest that the enhanced syntheses of beta and beta' subunits of RNA polymerase and of p factor observerd represent "escape synthesis", resulting from the close linkage of the prophage genome to the respective structural genes. In contrast, omega factor synthesis was stimulated upon induction of any of the lysogens used without respect to the site of prophage location, suggesting the involvement of an entirely different mechanism.

Induction of sigma factor synthesis in Escherichia coli by the N gene product of bacteriophage lambda

Thermoinduction of cells of E. coli carrying prophage lambdacI857 within the bfe gene brings about not only "escape synthesis" of core subunits of the DNA-dependent RNA polymerase (RNA nucleotidyltransferase, nucleosidetriphosphate:RNA nucleotidyltransferase, EC 2-7-7-6), but also a striking stimulation of sigma factor synthesis. The latter phenomenon, termed sigma induction, is generally observed after lambda phage infection or prophage induction. A series of experiments with various bacterial and phage strains led us to conclude that the N gene product of lambda is directly involved to the sigma induction. These and other results obtained with mutants defective in transcription termination factor rho suggest the involvement of a rho-sensitive site in the control of sigma gene expression in E. coli.

In vitro transcription of the tryptophan operon in isolated bacterial nucleoids

In vitro transcription of the trp operon in isolated nucleoids from Escherichia coli was studied. RNA synthesis in this system occurred primarily as a continuation of transcription which had been initiated in vivo; little or no initiation of new RNA chains was observed. Transcription of the trp operon in nucleoids by endogenous RNA polymerase procedded efficiently and ceases sequentially in the order of the gene sequence within the operon. Under these conditions, no appreciable exonuccleolytic digestion of nascent 3H-RNA was found, though some endonucleolytic cleavage was generally seen. Little or no incorporation of 14C-leucine into polypeptides was observed, inspite of tha fact that considerable number of ribosomes and nascent RNA chains were found attached to the isolated nucleoids. The synthesis of trp mRNA continued in the presence of chloramphenicol or fusidic acid, or under conditions where the rebosomal translocation factor G was inactivated. From these and other kinetic studies of trp mRNA synthesis in nucleoids obtained from nonsense strong polar mutants of the trp operon, it was shown that transcription in nucleoids was not connected functionally with transloational processes and thus unable to exhibit polarity effected by a nonsense mutation or by general translational blockage. In studies employing nucleoids from nonsense strong polar mutants of the trp operon, it was demonstrated that RNA polymerase are scantily distributed over the region downstream from the nonsense mutation site of the operon, thereby supporting a notion that in vivo transcription is eventually terminated near the nonsense mutation.

Efficient suppression of the requirement for N function of bacteriophage lambda by a Rho-defective E.coli suA mutant

The E. coli suA mutant (T82), which is a suppressor of polarity by virtue of its impaired transcription termination factor (rho) activity, is shown to be an efficient suppressor of lambdaN- mutants. The relief of the N requirement by this host is reflected in a substantial restoration of growth and N-dependent beta-galactosidase expression as well as in a partial relief of polarity of N-defective phages.

Suppression of polarity of insertion mutations in the gal operon and N mutations in bacteriophage lambda

Bacterial mutations (psuA and psu) known for their ability to suppress the polarity on nonsense mutations are shown to suppress the polarity of certain insertion mutations in the gal operon. The short insertion, IS1 (800 nucleotide pairs), is about 15 to 50% suppressed, whereas longer insertions, IS2 (1,400 nucleotide pairs), and IS3 (1,200 nucleotide pairs), are not. Some of the polarity suppressor mutations (psu-1, psu-2, and psu-3) are at least partially permissive for N-gene mutations (N7 and N53) of bacteriophage lambda, suggesting a relationship between natural and mutational polar signals. That this relationship may be complex is indicated by the fact that other suppressor mutations, effective in suppressing nonsense or insertion polarity, fail entirely to permit the growth of lambda N mutants.