Isolation and mapping of plasmids containing the Salmonella typhimurium origin of DNA replication

Escherichia coli cells lacking methylation-blocking factor (leucine-responsive regulatory protein) have precise timing of initiation of DNA replication in the cell cycle

A protein that is required for specific methylation inhibition of two GATC sites in the papBA pilin promoter region, known as methylation-blocking factor (Mbf) and recently shown to be identical to the leucine-responsive regulatory protein (Lrp), is not responsible for the delayed methylation at oriC implicated in an eclipse period following initiation of DNA replication. Cells containing a transposon mutation within the mbf (lrp) gene initiate DNA replication at the correct time during the cell cycle, whereas cells with increased amounts of the Dam methyltransferase initiate DNA replication randomly throughout the cell cycle.

Characterization of an autonomously replicating region from the Streptomyces lividans chromosome

The chromosomal replication origin of the plasmidless derivative (TK21) from Streptomyces lividans 66 has been cloned as an autonomously replicating minichromosome (pSOR1) by using the thiostrepton resistance gene as a selectable marker. pSOR1 could be recovered as a closed circular plasmid which shows high segregational instability. pSOR1 was shown to replicate in Streptomyces coelicolor A3(2) and in S. lividans 66 and hybridized with DNA from several different Streptomyces strains. Physical mapping revealed that oriC is located on a 330-kb AseI fragment of the S. coelicolor A3(2) chromosome. DNA sequence analyses showed that the cloned chromosomal oriC region contains numerous DnaA boxes which are arranged in two clusters. The preferred sequence identified in the oriC region of Escherichia coli and several other bacteria is TTATCCACA. In contrast, in S. lividans, which has a high GC content, the preferred sequence for DnaA boxes appears to be TTGTCCACA.

Pseudomonas chromosomal replication origins: a bacterial class distinct from Escherichia coli-type origins

The bacterial origins of DNA replication have been isolated from Pseudomonas aeruginosa and Pseudomonas putida. These origins comprise a second class of bacterial origins distinct from enteric-type origins: both origins function in both Pseudomonas species, and neither functions in Escherichia coli; enteric origins do not function in either pseudomonad. Both cloned sequences hybridize to chromosomal fragments that show properties expected of replication origins. These origin plasmids are highly unstable, are present at low copy number, and show mutual incompatibility properties. DNA sequence analysis shows that both origins contain several 9-base-pair (bp) E. coli DnaA protein binding sites; four of these are conserved in position and orientation, two of which resemble the R1 and R4 sites of the E. coli origin. Conserved 13-bp direct repeats adjacent to the analogous R1 site are also found. No GATC sites are in the P. aeruginosa origin and only four are in the P. putida origin; no other 4-bp sequence is present in high abundance. Both origins are found between sequences similar to the E. coli and Bacillus subtilis dnaA, dnaN, rpmH, and rnpA genes, a gene organization identical to that for B. subtilis and unlike that of E. coli. A second autonomously replicating sequence was obtained from P. aeruginosa that has some properties of bacterial origins.

Methylation of GATC sites is required for precise timing between rounds of DNA replication in Escherichia coli

We have used the Koppes and Nordstrøm (Cell 44:117-124, 1986) CsCl density transfer approach for analysis of DNA from exponentially growing, isogenic Escherichia coli dam+ and dam mutant cells to show that timing between DNA replication initiation events is precise in the dam+ cells but is essentially random in the dam cells. Thus, methylation of one or more GATC sites, such as those found in unusual abundance within the origin, oriC, is required for precise timing between rounds of DNA replication, and precise timing between initiation events is not required for cell viability. Both the dam-3 point mutant and the delta(dam)100 complete deletion mutant were examined. The results were independent of the mismatch repair system; E. coli mutH cells showed precise timing, whereas timing in the isogenic E. coli mutH delta(dam)100 double mutant was random. The mechanism is thus different from the role of Dam methylation in mismatch repair and probably involves conversion of hemimethylated GATC sites present in daughter origins just after initiation to a fully methylated state.

The Escherichia coli dam gene is expressed as a distal gene of a new operon

DNA containing the Escherichia coli dam gene and sequences upstream from this gene were cloned from the Clarke-Carbon plasmids pLC29-47 and pLC13-42. Promoter activity was localized using pKO expression vectors and galactokinase assays to two regions, one 1650-2100 bp and the other beyond 2400 bp upstream of the dam gene. No promoter activity was detected immediately in front of this gene; plasmid pDam118, from which the nucleotide sequence of the dam gene was determined, is shown to contain the pBR322 promoter for the primer RNA from the pBR322 rep region present on a 76 bp Sau3A fragment inserted upstream of the dam gene in the correct orientation for dam expression. The nucleotide sequence upstream of dam has been determined. An open reading frame (ORF) is present between the nearest promoter region and the dam gene. Codon usage and base frequency analysis indicate that this is expressed as a protein of predicted size 46 kDa. A protein of size close to 46 kDa is expressed from this region, detected using minicell analysis. No function has been determined for this protein, and no significant homology exist between it and sequences in the PIR protein or GenBank DNA databases. This unidentified reading frame (URF) is termed urf-74.3, since it is an URF located at 74.3 min on the E. coli chromosome. Sequence comparisons between the regions upstream of urf-74.3 and the aroB gene show that the aroB gene is located immediately upstream of urf-74.3, and that the promoter activity nearest to dam is found within the aroB structural gene. This activity is relatively weak (about 15% of that of the E. coli gal operon promoter). The promoter activity detected beyond 2400 bp upstream of dam is likely to be that of the aroB gene, and is 3 to 4 times stronger than that found within the aroB gene. Three potential DnaA binding sites, each with homology of 8 of 9 bp, are present, two in the aroB promoter region and one just upstream of the dam gene. Expression through the site adjacent to the dam gene is enhanced 2- to 4-fold in dnaA mutants at 38 degrees C. Restriction site comparisons map these regions precisely on the Clarke-Carbon plasmids pLC13-42 and pLC29-47, and show that the E. coli ponA (mrcA) gene resides about 6 kb upstream of aroB.

Kinetic analysis for optimization of DNA ligation reactions

Kinetic equations describing ligation of DNA to circular recombinant forms were developed and solved for four types of reactions: (a) a homogeneous population of singly restricted DNA fragments, (b) insertion of singly restricted insert into vector, (c) forced directional insertion of doubly restricted insert into vector, and (d) insertion of singly restricted insert into phosphatased vector. The effects of varying vector and insert sizes, starting concentrations, and phosphatase treatment on the yield of circular 1:1 recombinants were analyzed. Selected theoretical predictions were experimentally tested and verified. Our suggestions on optimizing ligation reactions in several cases are at variance with common practice. For example, optimum conditions in case (b) and (d) ligations are best specified as individual insert and vector concentrations rather than as insert/vector molar ratios, except in case (d) ligations involving very small insert size. In case (c) ligations, highest efficiencies are obtained when both vector and insert are at relatively low concentration.

Klebsiella pneumoniae origin of replication (oriC) is not active in Caulobacter crescentus, Pseudomonas putida, and Rhodobacter sphaeroides

A DNA fragment carrying genes encoding the conjugal transfer system of the broad host range plasmid RK2 was inserted into a plasmid carrying the chromosomal origin of replication (oriC) from Klebsiella pneumoniae. The resulting plasmid, pEON1, was readily transferred between gram-negative bacteria and carried two potential origins of replication: oriC and the replication origin from pBR322 (oriPBR). Although pEON1 could be transferred to Caulobacter crescentus, Pseudomonas putida, and Rhodobacter sphaeroides, pEON1 was not maintained in these strains. However, an oriC-containing plasmid was maintained in these nonenteric bacteria when an RK2 origin of replication was present on the plasmid. Thus, the inability of pEON1 to be established in a nonenteric bacterium represents a failure of oriC to function as an origin of replication rather than a toxic effect of oriC. The initiation potential of the chromosomal origin of replication from K. pneumoniae appears to be realized only in enteric bacteria.

Cell cycle-specific replication of Escherichia coli minichromosomes

The timing of Escherichia coli minichromosome replication in the cell division cycle was examined using an improved procedure for studying plasmid replication frequency. Cultures growing exponentially in glucose/Casamino acids minimal medium were pulse-labeled with [3H]thymidine, and the radioactivity incorporated into plasmid DNA in cells of different ages was analyzed. At the end of the labeling period the bacteria were bound to the surface of a nitrocellulose membrane filter, and the radioactivity in new daughter cells, which eluted continuously from the membrane, was quantitated following agarose gel electrophoresis. The minichromosomes replicated during a discrete interval in the cell division cycle that appeared to coincide with initiation of chromosome replication. In contrast, plasmid pBR322 replicated throughout the division cycle at a rate that increased gradually as a function of cell age. The difference in minichromosome and pBR322 replication was clearly discernible in cells harboring both plasmids. It was also found that the 16 kD gene adjacent to oriC was not a determinant of the timing of minichromosome replication during the division cycle. The results are consistent with the conclusion that minichromosome replication frequency is governed by the same mechanism that controls chromosome replication.

Transcription termination within the Escherichia coli origin of DNA replication, oriC

Initiation of DNA replication from the Escherichia coli origin, oriC, is dependent on an RNA polymerase-mediated transcription event. The function of this RNA synthetic event in initiation, however, remains obscure. Since control of the synthesis of this RNA could serve a key role in the overall initiation process, transcription regulatory sites within and near oriC were identified using the galK fusion vector system. Our results confirm the existence of a transcription termination signal within oriC, first identified by Hansen et al. (1981), for the 16 kd transcript that is transcribed counterclockwise towards oriC. Termination is shown to be 92% efficient. A similar approach led to the detection of transcription termination within the chromosomal replication origin of Klebsiella pneumoniae. Approximately 50% of the E. coli 16 kd transcripts appear to terminate before reaching oriC between the XhoI (+416 bp) and the HindIII (+243 bp) sites. The predominant 3' ends of RNA that enter oriC, as determined by SI nuclease mapping, were located at positions +20 +/- 2, +23 +/- 2, +37, +39, +52, +66, +92, and +107. These termination sites, which map cl to RNA . DNA junctions identified by Kohara et al. (1985), appear as triplets and quadruplets. The E. coli oriC Pori-L promoter described in in vitro transcription studies by Lother and Messer (1981) was not detected in this study in either wildtype cells or isogenic dnaA mutants at the nonpermissive temperature. A new promoter activity, Pori-R1, was identified within the E. coli origin in the clockwise direction.

dnaA protein-regulated transcription: effects on the in vitro replication of Escherichia coli minichromosomes

Both initiation of replication and initiation of transcription are influenced by dnaA protein, when minichromosomes are assayed in vitro for dnaA protein complementation. This dnaA protein effect is seen only if minichromosomes are used containing the 16-kd promoter, from which transcription is directed into the minimal origin. Determination of the 16-kd promoter activity both in vivo and in vitro showed that this strong promoter is specifically repressed by dnaA protein. The 16-kd promoter is thus an integral regulatory region of oriC.

Overproduction of subunit a of the F0 component of proton-translocating ATPase inhibits growth of Escherichia coli cells

A hybrid plasmid, pKY159, carrying the promoter and the proximal region of the gene cluster for proton-translocating ATPase caused growth inhibition of Escherichia coli cells (K. Yamaguchi and M. Yamaguchi, J. Bacteriol. 153:550-554, 1983). The mechanism of this growth inhibition was studied, especially in terms of the responsible gene(s). Insertion of IS1, IS5, or gamma delta between the promoter and the gene for a possible component of the ATPase of 14,000 daltons (14K protein) released the inhibitory effect by pKY159. Deletion of the gene for subunit a also released the effect. However, deletion in the gene for the 14K protein released the effect only with an additional insertion within the gene. These results suggested that overproduction of subunit a is closely related to growth inhibition, whereas the 14K protein is not.

Insertions of transposable elements in the promoter proximal region of the gene cluster for Escherichia coli H+-ATPase: 8 base pair repeat generated by insertion of IS1

A plasmid pKY159 (Yamaguchi and Yamaguchi 1983) carrying a promoter proximal portion of the gene cluster of the proton-translocating ATPase (H+-ATPase) of Escherichia coli causes growth inhibition of wild-type cells. Insertion of a transposable element in this plasmid released this inhibitory effect. In analyzing this inhibitory effect, we determined the insertion points at the nucleotide-sequence level of transposable elements on 30 independent derivatives of pKY159 . Insertions of IS1, IS5, and gamma delta were found between the promoter and the gene for a possible component of 14,000 daltons of the H+-ATPase. Of 31 insertions, 26 were of IS1 and were located at the same site, indicating that this site is a hotspot for IS1 insertion and that IS1 insertion is much more frequent than that of IS5 or gamma delta in this region. Four different sites for IS1 insertion were found; in two of these an 8 base pair (bp) duplicate of the target sequence ( AAAAACGT and AAACGTTG ) was generated, while in the other two a 9 bp duplicate was found. In all cases in this study the nucleotide sequence of IS1 was the same as that of IS1-K. In the two cases with an 8 bp duplicate in different sites, a common 6 bp sequence ( AAACGT ) was found. These results suggested that generation of the 8 bp duplicate is related to the common sequence rather than a mutation in IS1 suggested by Iida et al. (1981) and also suggested that the essential length of the duplicate is 8 bp or less than 8 bp. A 6 bp sequence ( GTGATG ) homologous to the end portion of IS1 was found at the hotspot , but not at other sites, suggesting that this homology contributed to the high frequency of IS1 insertion.(ABSTRACT TRUNCATED AT 250 WORDS)

Enzymology of DNA in replication in prokaryotes

This review stresses recent developments in the in vitro study of DNA replication in prokaryotes. New insights into the enzymological mechanisms of initiation and elongation of leading and lagging strand DNA synthesis in ongoing studies are emphasized. Data from newly developed systems, such as those replicating oriC containing DNA or which are dependent on the lambda, O, and P proteins, are presented and the information compared to existing mechanisms. Evidence bearing on the coupling of DNA synthesis on both parental strands through protein-protein interactions and on the turnover of the elongation systems are analyzed. The structure of replication origins, and how their tertiary structure affects recognition and interaction with the various replication proteins is discussed.

Maintenance and incompatibility of plasmids carrying the replication origin of the Escherichia coli chromosome: evidence for a control region of replication between oriC and asnA

Plasmids that replicate only by means of the cloned Escherichia coli replication origin (oriC) are called minichromosomes or oriC-plasmids. In this paper it is shown that sequences located between oriC and asnA are involved in maintenance and incompatibility of minichromosomes. These sequences include part of the 16kD and 17kD genes, previously allocated within this region (1,2). Transcription towards oriC that is initiated at the 16kD promoter, specifically enhances the stability and copy-number of minichromosomes. Three regions are involved in minichromosome incompatibility. One region, incA, includes the minimal oriC sequence. A second, incB, maps within a 210 base pairs fragment that overlaps the 16kD promoter. The third, incC, encompasses the 17kD gene. Neither one of the regions expresses incompatibility on its own, but the additional presence of one of the others is required. The data presented indicate that sequences of the 16kD and 17kD genes are part of the replication control system of oriC-plasmids.

Chromosomal replication origin from the marine bacterium Vibrio harveyi functions in Escherichia coli: oriC consensus sequence

The chromosomal replication origin (oriC) of Vibrio harveyi has been isolated on a plasmid and shown to function as an origin in Escherichia coli. The nucleotide sequence of the V. harveyi oriC was determined. From a comparison of this sequence with oriC sequences of five enteric bacteria, we derived a consensus sequence of bacterial origins that function in E. coli. This consensus sequence identifies 122 positions within oriC where nucleotide substitutions can occur without loss of origin function. These positions are clustered rather than scattered. Four interrelated nine-base-pair repeats and eight of the dam methylation G-A-T-C sites are conserved in the consensus sequence. Very few relative insertion-deletion changes occur, and these are localized to one region of oriC. The genes for three polypeptides linked to the V. harveyi oriC were identified by using in vitro protein synthesis directed by deletion derivative plasmid templates. One of these genes, coding for a 58,000 Mr polypeptide and located 3.0 kilobase pairs from the V. harveyi oriC region, is lethal to E. coli when many copies (approximately 40 per cell) are present (high copy lethal or HCL gene). In addition, nucleotide sequence analysis showed that a different gene, the gid gene to the left of oriC, is highly conserved between E. coli and V. harveyi, whereas the coding region to the right of oriC is much less conserved.

Chromosomal replication origins (oriC) of Enterobacter aerogenes and Klebsiella pneumoniae are functional in Escherichia coli

The chromosomal DNA replication origins (oriC) from two members of the family Enterobacteriaceae, Enterobacter aerogenes and Klebsiella pneumoniae, have been isolated as functional replication origins in Escherichia coli. The origins in the SalI restriction fragments of 17.5 and 10.2 kilobase pairs, cloned from E. aerogenes and K. pneumoniae, respectively, were found to be between the asnA and uncB genes, as are the origins of the E. coli and Salmonella typhimurium chromosomes. Plasmids containing oriC from E aerogenes, K. pneumoniae, and S. typhimurium replicate in the E. coli cell-free enzyme system (Fuller, et al., Proc. Natl. Acad. Sci. U.S.A. 78:7370--7374, 1981), and this replication is dependent on dnaA protein activity. These SalI fragments from E. aerogenes and K. pneumoniae carry a region which is lethal to E. coli when many copies are present. We show that this region is also carried on the E. coli 9.0-kilobase-pair EcoRI restriction fragment containing oriC. The F0 genes of the atp or unc operon, when linked to the unc operon promoter, are apparently responsible for the lethality.

An amplified chromosomal sequence that includes the gene for dihydrofolate reductase initiates replication within specific restriction fragments

We have developed a methotrexate-resistant Chinese hamster ovary cell line (CHOC 400) containing a 500-fold amplification of a 135-kilobase chromosomal DNA sequence. This sequence includes the gene for dihydrofolate reductase (tetrahydrofolate dehydrogenase, 5,6,7,8-tetrahydrofolate: NADP+ oxidoreductase, EC 1.5.1.3). The high copy number of the amplified sequence permits it to be visualized as a distinct series of restriction fragments in genomic digests separated on ethidium bromide-stained agarose gels. Initiation of DNA replication in the amplified sequence was studied by radiolabeling DNA synthesized during the onset of S phase in synchronized CHOC 400 cells. Autoradiography of Southern blots of labeled genomic digests shows that DNA synthesis initiates in a small subset of the EcoRI fragments derived from the amplified units. These early labeled fragments are not synthesized at later times during S phase, when different subsets of fragments are synthesized. Regardless of the drug used to collect cells at the beginning of S phase, the replication pattern observed remains the same. These data suggest that replication of the amplified sequence initiates at specific sites within each repeated unit and proceeds in nonrandom order throughout the remainder of the sequence--i.e., that initiation of DNA synthesis in the chromosomes of mammalian cells is sequence specific.

Primary structure of the chromosomal origins (oriC) of Enterobacter aerogenes and Klebsiella pneumoniae: comparisons and evolutionary relationships

The nucleotide sequences of the Enterobacter aerogenes and Klebsiella pneumoniae DNA replication origins (oriC) were determined and compared with those of Escherichia coli and Salmonella typhimurium. Four interrelated, 9-base-pair repeats were identified from the conserved regions within the minimal origin. Evolutionary rates calculated from the minimal origin sequences yielded a quantitative phylogenic tree which agreed with the taxonomic classification of these genera.

The chromosomal origin of replication (oriC) of Erwinia carotovora

The chromosomal DNA replication origin (oriC) of the plant pathogen Erwinia carotovora has been isolated and sequenced. The minimal E. carotovora oriC regional functional in Escherichia coli is a 374 base pair region located on a 7.9 kilobase pair SalI fragment which also contains a functional asnA gene. Differences between the nucleotide sequence of the minimal origin regions of E. carotovora and those of E. coli and Salmonella typhimurium are clustered nucleotide substitutions, with regions of complete homology, up to 19 base pairs long, between the three origins. Nine GATC sites are found in the minimal origin, and all are conserved. In contrast, the region toward asnA from the minimal origin shows little clustering and the differences occur mainly every third nucleotide, suggesting that this region is a protein coding region.

Kinetics of minichromosome replication in Escherichia coli B/r

Replication control of the minichromosome pAL2 was found to differ from that of the chromosome in synchronously dividing populations of Escherichia coli B/r. Initiation of minichromosome replication took place at an increasing rate throughout synchronous growth. No coupling to initiation of chromosome replication was detected. Minichromosome replication was further examined in a dnaA5(Ts) temperature-sensitive initiation mutant. When cultures held at nonpermissive temperature (41 degrees C) for 60 min were shifted to permissive temperature (25 degrees C), initiation of both pAL2 and chromosome replication ensued in two waves spaced 25 to 35 min apart. Evidence is presented that minichromosomes terminate replication by passing slowly through a series of dimeric intermediate forms before reaching the closed circular monomeric form. The consequence of this slow passage as a rate-limiting step in the initiation reaction is discussed.

Structure and function of the region of the replication origin of the Bacillus subtilis chromosome. I. Isolation and characterization of plasmids containing the origin region

A BamHI restriction endonuclease fragment, B7, which is replicated first among all other fragments derived from the Bacillus subtilis chromosome, was cloned in Escherichia coli using as vector a hybrid plasmid pMS102 that can replicate both in E. coli and B. subtilis. Digestion of pMS102 with BamHI produced two fragments and the smaller one was replaced by the B7 fragment. The cloned plasmid pMS102'-B7 exhibited some peculiar properties that were not observed with plasmids containing other fragments from the B. subtilis chromosome. (1) E. coli cells harboring this plasmid stuck to each other and to glass. This property was more apparent when cells were grown in poor media. (2) E. coli cells tended to lose the plasmid spontaneously when they were grown without the selective pressure favorable to the plasmid. (3) The frequency of transformation of B. subtilis by pMS102'-B7 was less than 1/1,000 of that by the vector plasmid pMS102'. The number of copies of pMS102'-B7 present in the transformants was also markedly reduced, although the pUB110 origin of replication on the vector was intact and should be functional in B. subtilis. This inhibitory effect of the B7 fragment on plasmid replication was confirmed more directly by developing a semi in vitro replication system using protoplasts. Both in E. coli and B. subtilis the B7 fragment affected replication of its own molecule but not that of the coexisting plasmid with an identical replication system. The implication of the function of the B7 fragment in the initiation of the B. subtilis chromosome will be discussed.

Eukaryotic DNA segments capable of autonomous replication in yeast

A selective scheme is presented for isolating sequences capable of replicating autonomously in the yeast Saccharomyces cerevisiae. YIp5, a vector that contains the yeast gene ura3, does not transform a ura3 deletion mutant to Ura+. Hybrid YIp5-Escherichia coli DNA molecules also fail to produce transformants. However, collections of molecular hybrids between YIp5 and DNA from any of six eukaryotes tested (S. cerevisiae, Neurospora crassa, Dictyostelium discoideum Ceanhorabditis elegans, Drosophila melanogaster, and Zea mays) do transform the deletion mutant. The Ura+ transformants grow slowly, are unstable under nonselective conditions, and carry the transforming DNA as autonomously replicating, supercoiled circular molecules. Such a phenotype is qualitatively identical to that of strains transformed by molecules containing a yeast chromosomal origin of replication. Thus, these DNA hybrid molecules may contain eukaryotic origins of replication. The isolated sequences may be useful in determiing the signals controlling DNA replication in yeast and in studying both DNA replication and transformation in other eukaryotic organisms.

Copy-number mutants of the plasmid carrying the replication origin of the Escherichia coli chromosome: evidence for a control region of replication

A composite plasmid (pXX11) was constructed by joining of an oriC plasmid (pMCR115) carrying the replication origin (oriC) of the Escherichia coli chromosome and a mini-F plasmid (pSC138) carrying the ampicillin-resistance gene (bla). Plasmid pXX11 can replicate, by using oriC, in Hfr cells and mafA mutant cells that cannot support replication of an F plasmid. This plasmid is stably maintained in these host cells during cell growth even under nonselective conditions by use of the partition mechanism of the mini-F genome. In contrast to other oriC plasmids reported previously, pXX11 has no detectable effect on host cell growth. Higher copy-number (Cop-) mutants of pXX11 were isolated, and some of them were found to carry an insertion or deletion within a region derived from the E. coli chromosome. This region, designated cop (copy number), covers about 0.7 kilobase pair and is located approximately 3 kilobase pairs away from the oriC region at the side opposite the asn gene. Evidence suggests that the normal cop region locted on the oriC plasmid acts to reduce the copy number of the plasmid. Plasmid pXX11 complements the uncB402 mutation located on the host chromosome, but some of the Cop- plasmids do not, suggesting that the cop region is vey closely linked to uncB.

Nucleotide sequence of the Salmonella typhimurium origin of DNA replication

Construction of deletion derivative plasmids and cloning of restriction fragments from plasmids containing the Salmonella typhimurium origin of replication (ori) were used to locate the functional origin to within a DNA fragment of 296 base pairs between the genes uncB and asn. The nucleotide sequence of the S. typhimurium ori region was determined and compared with the Escherichia coli ori sequence. In the 296-base pair fragment, 85.8% of the bases are conserved between the two species. A nearly equal number of transition and transversion type differences, with no insertions or deletions, occurs between the two bacterial origins, such that the relatively high percentage (adenine plus thymine) of 59.5% is conserved. The 296-base pair fragment contains 14 GATC sequences, all of which are conserved. The high frequency of occurrence of GATC, which is the site of methylation under control of the dam gene, may explain in part why the bacterial ori region appears to be so highly conserved. A large number of secondary structures are possible. One such structure, with a "cloverleaf," is favored by ori nucleotide sequence comparisons and leads to potential novel macromolecular interactions.

Mutations in two unlinked genes are required to produce asparagine auxotrophy in Escherichia coli

Escherichia coli K-12 has two genes, asnA+ and asnB+, either one of which is able to satisfy the need of cells for asparagine. In order for a strain to have an auxotrophic requirement for asparagine, both genes must be mutationally inactivated. We obtained mutants with Tn5 inserted in asnB. asnB was mapped by conjugation and by three-factor P1 transductions at 15 min on the E coli K-12 linkage map, between ubiF and nagB. Specialized transducing phage lamba 781 supE was shown to carry asnB, as well as supE, ubiF, nagA, and nagB. asnA is the previously mapped ilv-linked asn locus, whiich is between uncB and rbs. E. coli C also has two asn genes, corresponding to asnA and asnB.