PubMed Health⌕ Search

Biomedical subjects

K C Luk

Publications and source records attributed to K C Luk.

35 records · Page 2Linked to original sources
PubMed1983

Tandem transcription-termination sites in the late rightward operon of bacteriophage lambda.

A transcription termination site (designated as t'R2) is located between the rightward late t'R1 terminator and the S gene of phage lambda. This t'R2 terminator is rightward and absolutely dependent on the rho factor, being about 45% effective in rho+ E. coli and only 6% in rho- cells at 30 degrees C. This 7.5-fold rho dependence of t'R2 is in contrast to that of tR1 (4.5 fold, from about 81% to 18%) and the partial rho dependence of t'R1 (1.4 fold, from 96% to 67%). At the elevated temperature of 42 degrees C, t'R2 becomes 1.5 times more leaky (with about 2.5-fold reduction in termination efficiency) than tR1 or t'R1 (with only 1.1-fold reduction) in rho+ hosts. The calculated joint efficiencies of t'R1 and t'R2 are 98% in rho+ cells at 30 degrees C. t'R2 is also active in vitro, but only in the presence of rho factor, whereas t'R1 is active both in the presence and absence of rho. However, the in vitro termination at t'R1 is enhanced about 1.7-fold by the rho factor. The properly oriented lambda nutR site together with the N gene function bring about almost complete antitermination at t'R2 (96% effective), but incomplete at t'R1 (72%). The termination points at t'R2 are located around 532-534 bp to the right of the s'R startpoint of the p'R-initiated RNA on lambda DNA (or 338-340 bp downstream of t'R1) and 66-68 bp to the left of the S gene, as determined by S1 mapping. The t'R2 termination points are located within a dyad symmetry region which, in the transcript, is able to form a hairpin structure consisting of 16 bp in the stem and 6 bases in the loop. It is proposed that t'R2 acts as a second terminator to block any readthrough transcription initiated at the late promoter p'R into the late genes of phage lambda.

Bacteriophage lambda↗
PubMed1983

The tL2 cluster of transcription termination sites between genes bet and ral of coliphage lambda.

The major leftward transcription of bacteriophage lambda is controlled by several terminators (t), including tL1, tL2, tL3, and others. The tL2 termination site, which was placed by Salstrom and Szybalski (Virology 88, 252-260, 1978) between lambda genes bet and ral, was found to consist of a cluster of four leftward terminators. As not to change the numbering of other leftward terminators, these were designated as tL2a, tL2b, tL2c, and tL2d. As determined by S1 nuclease mapping of the tL2-terminated in vitro transcripts, the normally pL-initiated major leftward lambda transcription should encounter termination points at 1653 bp (tL2a; between genes Ea10 and ral), 2089 bp (tL2b; between genes cIII and Ea10), 2441-2442 bp, and 2483 bp (tL2c and tL2d; both within gene gam) from the sL startpoint (= +1). All terminators were cloned in a pBR322-derived plasmid between the p'R promoter and the galK gene, and their in vivo termination efficiencies are 69% (tL2a), 53% (tL2b), and 38% (tL2c + tL2d), measured as reduction of galK expression in rho+galK- hosts at 30 degrees. The tL2a and tL2b), terminators depend little on the rho factor, whereas the efficiency of tL2c + tL2d decreases from 38 to only 14% in the rho- host. When shifted to 42 degrees, the termination efficiency of tL2b decreases from 53 to only 36%, while the other tL2 terminators are much less affected by increasing the temperature. The calculated joint efficiency of the entire tL2 cluster is 90%, which is in perfect agreement with the 90% termination efficiency reported by Salstrom and Szybalski (1978) for tL2. However, the natural location of tL2c and tL2d within the actively translated gam gene may interfere with their termination function. Under in vitro conditions, tL2c and tL2d are active only in the presence of rho factor, whereas tL2a and tL2b do not require rho. The structure of the tL2 terminators resembles that of some other known termination sites: a perfect (8 bp for tL2a and tL2b) or imperfect (8-9 bp) dyad symmetry and a T6 (tL2a), T5 (tL2b), T4 (tL2c) or TTATT sequence (tL2d) toward the 3' end of the mRNA-like DNA strand.

Bacteriophage lambda↗
PubMed1983

A cluster of leftward, rho-dependent t'J terminators in the J gene of coliphage lambda.

While searching for the N-unresponsive terminator described by Honigman (Gene 13 (1981) 299-309), a 1680-bp DNA fragment from within gene J of bacteriophage lambda was cloned into plasmid pD12 between promoter p'R and the galK gene. In vitro transcription of this plasmid and S1 mapping assays, together with nucleotide sequencing, demonstrated that this DNA fragment contains a cluster of at least four in rho+ Escherichia coli hosts and only 30% in rho- hosts at at 30 degrees C. At the elevated temperature of 42 degrees C, the rho-dependent termination component of the t'J cluster becomes somewhat leaky, with the readthrough increasing by about twofold. The t'J terminators appear to be less responsive to nutR- and N-mediated antitermination (efficiency of 84-87%) than tL3, which responds to the same antitermination function with an efficiency of 97%. The relationship between the t'J cluster and the same antitermination function with an efficiency of 97%. The relationship between the t'J cluster and the highly N-unresponsive leftward termination signal tJ, which is also located within the J gene region (Gottesman et al., J. Mol. Biol. 140 (1980) 57-75; Honigman, Gene 13 (1981) 299-309), is unknown.

Bacteriophage lambda↗
PubMed1983

Effect of multiplicity of infection on transcription in Escherichia coli cells infected by bacteriophage lambda.

The effect of multiplicity of infection (m.o.i.) on transcription was studied by infecting Escherichia coli with bacteriophage lambda cI47, lambda cI47O29P3 and lambda cI857cro27P3. DNA-RNA hybridization with lambda cI47 l-strand DNA, phi 80imm lambda r-strand DNA, and lambda imm80 r-strand DNA were used to measure mRNA transcription from the l-strand, the r-strand of the early x-P-Q region and the late A-J-b2 region of lambda bacteriophage respectively. In lambda cI47cro+O-P--infected cells, transcription from the l-strand, r-strand of the early x-P-Q region and the late A-J-b2 region all decreased with increasing m.o.i. The response in the x-P-Q region was less marked than in other regions, but the pattern looked similar to that described above. When phage DNA replication was permitted, as in the case of lambda cI47, the response was similar to that observed in lambda cI47cro+O-P--infected cells, but the level of transcription was increased two- or threefold. In lambda cI857cro-P--infected cells, the leftward transcription and the rightward transcription from the early x-P-Q region and the late A-J-b2 region all increased with increasing m.o.i., but the extent of change was less drastic than with lambda cro+. This result demonstrated clearly that the decrease in transcription from various regions at increasing m.o.i. of lambda cro+ was due to the inhibitory action of the cro gene product. The results obtained with cro- strongly support the view that gene dosage is a significant controlling factor for the extent of gene expression.

Bacteriophage lambda↗
PubMed1982

A rho-independent termination caused by the cloned inverted nut L site of phage lambda.

For studying the termination activity of inverted nutL site of bacteriophage lambda, we have constructed a plasmid carrying the nutL fragment oriented reversely with respect to cloned lambda promoter p'R-directed transcription. The results of in vitro transcription on this plasmid template and S1 mapping assay reveal that the termination of p'R-promoted transcription at inverted nutL site is a rho-independent event. This nutL terminator shares several features with the other known sites of transcription termination, including (i) a uridine-rich 3' terminal RNA sequences,--UUAAUUUUU-OH, (ii) a GC-rich region in the DNA immediately preceding the site of termination, (iii) a region of dyad symmetry in the DNA which, in transcript, is capable of forming a stable hairpin containing four GC base pairs and one AU base pair in its stem.

Bacteriophage lambda↗
PubMed1982

Transcription termination: sequence and function of the rho-independent tL3 terminator in the major leftward operon of bacteriophage lambda.

For cloning, assaying the function and sequencing terminators, we have constructed the pD12 plasmid, in which the late promotor p'R of phage lambda controls the expression of the galK gene of the pK03 plasmid of McKenney et al. (1981). The lambda tL3 terminator region was cloned in this plasmid between the promoter and the galK gene, and found to be 90-94% effective in preventing galactokinase expression in both rho+ and rho- hosts. Is is also active in vitro, both in the presence or absence of the rho factor. The termination point is located at 4320 bp to the left of the SL startpoint of the PL-RNA, just downstream of gene exo. We have sequenced 356 bp of the hitherto uncharted lambda DNA to the right of the TaqI cut, which in turn is 110 bp to the right of the b522 deletion at 63.9% lambda. The tL3 terminator has several features common to other rho-independent termination sequences, including an 81% G+C-rich region of 2X8-bp symmetry ("stem") with a 5-bp intervening "loop", partially overlapping and followed by a sequence transcribed into the pyrimidine-rich CCUUUCU-OH 3' terminus of the RNA. The termination point that follows the last U was determined by the S1 mapping technique.

Bacteriophage lambda↗
PubMed1982

Toxic effects of chromic sulphate on the common carp, Cyprinus carpio.

The effects of chromic sulphate, which is commonly used in the tanning of leather, on the common carp, Cyprinus carpio and the bighead, Aristichthys nobilis, were studied. C. carpio was found to be more susceptible than A. nobilis to chromic sulphate at the lowest concentration tested (100 mg/l) whereas results obtained at other concentration were similar for both species. Damage to the gills, liver and intestine was observed by histopathological examination. A wide range of chemicals is used in the different stages of treating leather and a possible treatment of the effluent from the tanneries is suggested.

Animals↗
PubMed1982

Gene dosage as a regulatory factor for gene expression. I. In lambda plac5-infected cells.

To study the effect of gene dosage on gene expression, lambda plac5cI857O29P3, a replication defective lambda phage carrying part of the lac operon (containing the lac promotor, operator and z gene) in the b2 region was studied in Escherichia coli strain JC6256 where the lac operon is deleted and at a temperature where the lambda repressor is inactive. In measuring the synthesis of beta-galactosidase, it was possible to separate the effects of the lac promoter from those of the phage promoter. When the synthesis of beta-galactosidase was initiated from the inserted lac promoter in JC6256(lambda +) in the presence of additional cyclic AMP, the rate and level of beta-galactosidase synthesis were directly proportional to the multiplicity of infection (gene dosage). Furthermore, beta-galactosidase synthesis was initiated about 5 min after infection, just as with isopropyl-beta-D-thiogalactoside (IPTG) induction. When the synthesis of beta-galactosidase was initiated from the phage promoter in JC6256 in the absence of additional cyclic AMP, the rate and level of beta-galactosidase synthesis were again linearly proportional to gene dosage. On the other hand, initiation of beta-galactosidase synthesis was delayed until 10 to 20 min after infection. These results suggest that: (i) in the absence of negative controlling factors, the extent of gene expression is proportional to gene dosage; (ii) varying the gene dosage can be used to regulate gene expression.

Bacteriophage lambda↗
PubMed1980

The phage promoter responsible for the expression of the inserted beta-galactosidase gene in bacteriophage lambda plac5.

The lac transducing phage, lambda plac5, carries a segment of the E. coli lac operon on the left side of the b2 region of the lambda phage. In the absence of additional cyclic AMP, beta-galactosidase can only be expressed from the phage promoter, and the expression of the inserted lac promoter is suppressed. This phage promoter responsible for beta-galactosidase synthesis is shown to be under the control of the cI and N gene products; however, the repressive action of the cro gene product at high multiplicity of infection is not observed although some turn off at very late time is detected. To pin down this phage promoter, results described in this communication and those described elsewhere can rule out the promoter PI, PR, P'R, and the promoter PL also looks rather unlikely. No firm identification of this phage promoter has been made, but the promoter(s) in the b2 region (the b2 promoter) is proposed. The phage promoter responsible for beta-galacrosidase synthesis is shown to be a weak promoter, requires the Q gene product or one (or more) of the late gene products for activation, and the time of expression is very late.

Bacteriophage lambda↗
PubMed1979

The effect of digested sludge on the common carp, Cyprinus carpio L.

The effects of sewage sludge on the survival of the common carp, Cyprinus carpio, were investigated. It was noted that, when the concentrations of the sludge used for rearing carp exceeded 0.6%, the fish died within 42 days, whereas those reared in 0.2% sludge remained alive and healthy during the trial period. The body and the gills of the dead fish were covered by a veil-like film which looked like coagulated mucus and was formed by the heavy metal ions from the sludge reacting with some constituents of the mucus secreted by the gill. The histopathological symptoms of the various organs further demonstrated that internal injury in liver and kidney was also an important feature of the intoxicants as well as external damage, especially the particles observed around the gills. It was concluded that the optimum concentration of sludge to any species of fish should be determined by testing a series of concentrations before using it as a supplementary fish feed or fish pond fertilizer.

Animals↗
PubMed1977

Production of cyclopiazonic acid by Aspergillus flavus Link.

Production of cyclopiazonic acid by Aspergillus flavus is reported for the first time. A procedure for its production by agitated solid substrate fermentation on red wheat is described along with the isolation procedure and physical and chemical properties of this indole derivative. The compound has been found to exert antibacterial activity.

Aspergillus flavus↗
PubMed1977

The effects of zinc and copper salts on Cyprinus carpio and Ctenopharyngodon idellus.

The effects of zinc and copper salts on the survival of the two species of freshwater fish, common carp, Cyprinus carpio, and grass carp, Ctenopharyngodon idellus, were investigated. It was discovered that the levels of tolerance to the concentration of the metals were species-specific. Cyprinus carpio was found to be more susceptible to copper, whereas Ctenopharyngodon idellus was more susceptible to zinc. In general, copper was more toxic than zinc, as revealed by the survival times. The body and the gills of dead fish seemed to be covered by a veil-like film which looked like coagulated mucus and which was formed by the heavy-metal ions reacting with some constituents of the mucus and which was formed by the heavy-metal ions reacting with some constituents of the mucus secreted by the gill. The histopathological assessment of the gill and liver of Cyprinus carpio was also carried out. Particles were observed around the gills of the dead fish treated with zinc and copper salts, although no other major changes were found in the gill. Several histopathological changes were observed in the livers, including the presence of particles. The symptoms of the liver suggested that the internal injury was also an important feature of the intoxicants.

Animals↗
PubMedDate not supplied

Isolation and identification of "diazepam-like" compounds from bovine urine.

Attempts to isolate the putative endogenous ligand for the benzodiazepine receptor from bovine urine resulted in the identification of three isoflavans: equol (1), 3',7-dihydroxyisoflavan (2), and 4'-hydroxy-7-methoxyisoflavan (9), as "diazepam-like" compounds. 3-Chloro-9H-carbazole (17) was found to enhance the binding of diazepam in the benzodiazepine receptor binding assay. Pinosylvine monomethyl ether (18), indigo (20), and indirubin (21) were isolated as inactive compounds.

Animals↗