The DNA sequence of the phage lambda genome between PL and the gene bet

DNA-damaging agents induce the RecA-independent homologous recombination functions of integrating conjugative elements of the SXT/R391 family

Integrating conjugative elements (ICEs) of the SXT/R391 family are major contributors to the spread of antibiotic resistance genes. These elements also catalyze their own diversity by promoting inter-ICE recombination through the action of the RecA-independent homologous recombination system that they encode. Here, we report that expression of this recombination system, which consists of the single-stranded DNA annealing protein Bet and the exonuclease Exo, is induced by DNA-damaging agents via ICE-encoded transcriptional regulators. We show that the bet and exo genes are part of a large polycistronic transcript that contains many conserved ICE genes that are not involved in the main integration/excision and conjugative transfer processes. We show that although the recombination genes are highly transcribed, their translation is subject to additional strong regulatory mechanisms. We also show that an ICE-encoded putative single-stranded DNA binding protein (Ssb) limits hybrid ICE formation. Finally, a thorough in silico analysis reveals that orthologues of Bet and Exo are widely distributed in bacterial strains belonging to very distantly related bacterial species and are carried by various mobile genetic elements. Phylogenetic analyses suggest that the annealing proteins and exonucleases that compose these systems sometimes have different evolutionary origins, underscoring the strong selective pressure to maintain the functionality of these unrelated cooperating proteins.

Superinfection exclusion (sieB) genes of bacteriophages P22 and lambda

The superinfection exclusion gene (sieB) of Salmonella phage P22 was mapped with phage deletion mutants. The DNA sequence in the region was reexamined in order to find an open reading frame consistent with the deletion mapping. Several discrepancies with the previously published sequence were discovered. The revised sequence revealed a single open reading frame of 242 codons with six likely translation initiation codons. On the basis of deletion and amber mutant phenotypes, the second of these six sites was inferred to be the translation initiation site of the sieB gene. The sieB gene encodes a polypeptide with 192 amino acid residues with a calculated molecular weight of 22,442, which is in reasonable agreement with that estimated from polyacrylamide gels. The transcription start site of sieB was identified by the use of an RNase protection assay. The sieB promoter thus identified was inactivated by a 2-base substitution in its -10 hexamer. The sieB gene of coliphage lambda was also identified. The promoter for lambda sieB was identified by homology to that of P22 sieB.

The int genes of bacteriophages P22 and lambda are regulated by different mechanisms

Bacteriophage P22 and lambda are related bacteriophages with similar gene organizations. In lambda the cll-dependent Pl promoter is responsible for lambda int gene expression. The only apparent counterpart to pl in P22 is oriented in the opposite direction, and cannot transcribe the P22 int gene. We show that this promoter, called P(al), is active both in vivo and in vitro, and is dependent upon the P22 cll-like gene, called c1. We have also determined the DNA sequence of a 3.3 kb segment that closes the gap between previously reported sequences to give a continuous sequence between the P22 pL promoter and the int gene. The newly determined sequence is densely packed with genes from the pL direction, and the proteins predicted by the sequence show excellent correlation with the proteins mapped by Youderian and Susskind in 1980. However, the sequence contains no apparent genes in the opposite (p(al)) direction, and no additional binding motifs for the P22 c1 protein. We conclude that int gene expression in P22 is regulated by a different mechanism than in lambda.

Bacteriophage P22 accessory recombination function

The accessory recombination function (arf) gene of bacteriophage P22 is located immediately upstream of the essential recombination function (erf) gene. Three mutant alleles of arf were constructed and installed in P22 in place of the wild-type allele: an out-of-frame internal deletion, an in-frame internal deletion, and an amber mutation. The deletion mutant phages are partially defective in homologous recombination and plaque formation in wild-type and recA hosts; their defects are more severe in recB and recA recB hosts. The amber mutant phage exhibits the same growth phenotypes in nonsuppressing hosts, but not in an amber-suppressor host. Plasmids that express arf complement the growth defect of arf- phages. These plasmids stimulate erf-mediated recombination; they were also found to cause a small stimulation of recA-recBCD-mediated homologous recombination of phage lambda.

Genetic structure of the bacteriophage P22 PL operon

The sequence of 1416 base-pairs of the P22 PL operon was determined, linking a continuous sequence from PL through abc2. P22 mutants bearing deletions in the sequenced region were constructed and tested for their phenotypes. Plasmids were constructed to express PL operon genes singly and in combination from Plac UV5. Two previously known genes, 17 and c3, are located within this sequence. In addition, three new genes have been identified: ral, kil and arf. Genes ral and c3 are homologous, as well as functionally analogous, to lambda ral and cIII, respectively. P22 kil, like lambda kil, kills the host cell when it is expressed. The two kil genes, although analogous in cell killing and map location, have no apparent sequence homology. The functions of the P22 and lambda kil genes are unknown; however, P22 kil is essential for lytic growth in the absence of abc. Gene arf (accessory recombination function) is located just upstream from erf; it is essential for P22 growth in the absence of kil or other genes upstream in PL. The growth defect of P22 bearing a deletion that removes arf is complemented by expression of either arf or the lambda red genes from plasmids. Sequences that include the stop codon for gene 17 may form a small stem-loop structure and are nearly identical to lambda sequences that contain the stop codon for ssb, which is near lambda tL 2b. Plasmids that include the P22 structure negatively regulate kil gene expression in cis.

Modification enhancement by the restriction alleviation protein (Ral) of bacteriophage lambda

The product of the lambda ral gene alleviates restriction and enhances modification by the Escherichia coli K-12 restriction and modification system. An open reading frame (orf) located between genes N and Ea10 has been assigned to the ral gene. We have cloned this orf in a plasmid where its transcription is controlled by a thermolabile lambda repressor. Inactivation of the lambda repressor caused a 1000-fold reduction in K-specific restriction of unmodified lambda phage and a 100-fold increase in modification. In minicells transformed with ral+ plasmids, derepression resulted in the appearance of a polypeptide with a lower mobility than that predicted for a protein encoded by the orf attributed to ral; in a transcription and translation system in vitro DNA from a ral+ plasmid encoded a polypeptide with the same mobility. This polypeptide was absent when the plasmid DNA carried a mutant ral gene. The nucleotide sequence of this mutant gene defined two base changes, one of which inactivates the initiation codon of the orf. The K restriction endonuclease, which is also a K-specific methylase, is encoded by three genes designated hsdR, hsdM and hsdS, although the hsdR polypeptide is not essential for the methylase activity. We show that Ral enhances modification in a host strain lacking the entire hsdR gene, and lambda phages carrying the hsdM and S genes modify their own DNA inefficiently in the absence of Ral, despite the fact that derivatives of these phages provide efficient amplification of the K-specific methylase. Our data support a model in which, as a consequence of the interaction of Ral with either the hsdM or the hsdS polypeptide, the conformation of the enzyme is changed and the efficiency of methylation of unmodified target sites is enhanced. It has been postulated that Ral counteracts Rho, but in our experiments Ral did not relieve transcriptional polarity.

Translational regulatory signals within the coding region of the bacteriophage lambda cIII gene

Six independent mutations which enhance the lysogenic response were analyzed. The mutations cause single-base substitutions at three sites within the cIII coding sequence, one of which does not change the amino acid code. The mutations allow for elevated translation of the cIII gene, possibly via changes in the mRNA secondary structure.

Transcription termination and processing sites in the bacteriophage lambda pL operon

S1 nuclease mapping was performed on transcripts from the major leftward operon of the bacteriophage lambda in order to locate the 3' ends of stable RNA species produced in vivo. The analysis was carried out on RNA purified from either an induced lambda prophage or bacteria carrying a plasmid containing a large segment of lambda including the intact PL operon through the bet gene. The S1 nuclease mapping was performed on transcripts produced in the presence and the absence of the N antitermination function, and in the presence and the absence of either the RNase III processing enzyme or the Rho factor. The results of this work indicate that the intercistronic region between the N and ral genes of lambda contains three sites at which transcripts end under N-Rho+ conditions (positions on the lambda sequence: 34,826, 34,558 and 34,393). The distal two correspond to the two sites previously described in this region as tL1 (on both sides of the BamHI site). In the region between ral and Ea10, we mapped the 3' ends of three species of RNA. The 3' end of one species was found to be located 90 nucleotides proximal to tL2a, at 34,000 in the lambda sequence. The terminator at this site may be partially N-resistant. In an RNase III deficient host, an additional RNA species is formed. The 3' end of this RNA species is located at tL2a (33,910 on the lambda sequence). In the presence of the antitermination N gene product, the readthrough transcripts are processed to form a 3' end at position 33,980 on the lambda sequence. These results suggest that elongation of transcription of the lambda PL operon is reduced gradually by clusters of termination located between genes and that the expression of the terminated products is further controlled by processing of the mRNA.

Terminators of transcription with RNA polymerase from Escherichia coli: what they look like and how to find them

We present here a compilation of prokaryotic transcription terminator sequences (ref. 1-152). The compilation includes 49 independent terminators, 52 speculated independent terminators, 27 sites shown to function in vivo, and some 20 proven or speculated rho-dependent terminators. In addition to the well-known features of independent terminators (dyad symmetry and T-run), two consensus are found: CGGG(C/G) upstream and TCTG downstream of the termination point. A subset of the collection of sequence has been used to construct a computer algorithm to locate independent terminators by sequence analysis.

Selective inhibition of Escherichia coli recBC activities by plasmid-encoded GamS function of phage lambda

The gam locus of bacteriophage lambda encompasses two coding sequences with the same reading frame and translational stop, one corresponding to an Mr 11646 polypeptide (gamS gene), the other to an Mr 16349 polypeptide (gamL gene). A DNA segment encoding gamS but not gamL was placed under lambda pR promoter control (regulated by the cIts857-coded repressor) on a multicopy plasmid, and an insertion mutation (gamS201) was constructed. Expression of gamS+, but not gamS201, inhibited Escherichia coli RecBC nuclease in vivo; the criteria were inhibition of chromosomal DNA degradation after UV irradiation and plating of T4 gene 2- phages. The recB+ C+ bacteria expressing gamS+ were completely or partially similar to recC- mutants with respect to certain phenotypes: defective plating of phages P1 and P2, ability to plate (in a recA- background) lambda red- gam- phages, reduced resistance to UV irradiation, defective SOS induction, decreased colony-forming ability.

Initiator tRNA may recognize more than the initiation codon in mRNA: a model for translational initiation

A special methionine tRNA (tRNAi) is universally required to initiate translation. Amongst species a tRNAi structural conservation is most apparent in the anticodon and T arms of the molecule but extends into the variable loop and the 3' strand of the D stem. This suggested that they could share a similar ancestral or current function in initiation of translation. We report that the sequence of bases neighboring the translational start codons of many eubacterial genes are complementary not only to the extended anticodon but also to the D and T loops of tRNAi. Study of the coding properties of tRNAi and of mutations that affect translation suggests that the translational start domain can be a mosaic of signals complementary to the loops of tRNAi. The hypothesis of multiple loop recognition suggests that unusual triplets can start prokaryotic and mitochondrial genes and predicts the occurrence of other reading frames. Furthermore, it suggests a unifying model for chain initiation based on RNA contacts and displacements.

Conservation of genome form but not sequence in the transcription antitermination determinants of bacteriophages lambda, phi 21 and P22

Comparisons are made among DNA sequences upstream from terminators in both leftwards and rightwards early operons of related coliphages lambda, phi 21 and P22. These sequences include both left and right determinants of response to phage-coded antitermination proteins, "N", as well as the N structural genes themselves. Despite almost total disparity of DNA sequence, the three genomes can be discerned to include the same elements in the same order and spacing: downstream from the early left promoter are sequentially a site of recognition for host nusA protein, a dyad symmetry "nut" essential for N function in lambda, overlapping sites for processing of the transcript by RNAase III and then the N structural genes; downstream from the cro gene on the right are sites of nusA recognition and nut dyad symmetries homologous to those on the left. Because the N proteins of lambda, phi 21 and P22 do not for the most part complement each other, a specific site of N recognition has been postulated for each N-responding operon. The nut dyad symmetry qualifies as such a site, since the loop of the left dyad in lambda is marked by mutations that block N function leftwards, and since DNA sequences here show close homology between the loops of left and right dyads for each phage, but less if not little homology for different phages.

"N" transcription antitermination proteins of bacteriophages lambda, phi 21 and P22

Comparison is made among the amino acid sequences of three transcription antitermination proteins, based upon the DNA sequences of their genes in bacteriophages lambda, phi 21 and P22. The three proteins are all small (about 100 amino acids), hydrophilic and basic, but otherwise show little homology. A basic region near the amino terminus has several amino acid positions common to all three proteins and is the locus of mutations that alter six different amino acid positions inactivating the lambda N protein. A less basic region near the center is the locus of three mutations affecting the interaction of lambda N with host nusA protein. The N gene of phi 21 has an amino terminus more like that of P22, and a carboxy terminus clearly related to that of lambda.

Atypical deletions generated by mutated IS102 elements

The element IS102 potentially codes for two polypeptide chains. We have introduced several mutations in the larger one near the COOH terminus and determined the residual ability of the mutated elements to generate deletions in order to assign a role to this polypeptide chain. We show that in these elements, deletions still occur, although at a reduced level, but that in all cases examined so far the ends of the element are no longer recognized as the fixed endpoint of IS-mediated deletions, even though some other structural features of normal deletions formation are still present.

A computer algorithm for testing potential prokaryotic terminators

The nucleotide sequences of 30 factor-independent terminators of transcription with RNA polymerase from E. coli have been compiled and analyzed. The standard features - a stretch of thymine residues and a preceding dyad symmetry - are shared by most sequences, but there are striking exceptions which indicate that these features alone are not sufficient to describe these sites. In two thirds of the sequences the 3'-half of the dyad symmetry contains the pentanucleotide CGGG (G/C) or a close derivative; about one third have TCTG or a close derivative just downstream of the termination point. The TCTG -box might be implied in termination of stringently controlled operons of E. coli. An algorithm to locate terminators in templates of known nucleotide sequence has been constructed on the basis of correlation to the distribution of dinucleotides along the aligned signal sequences. The algorithm has been tested on natural sequences of a total length of about 11,500 N. It finds all known independent terminators and only a few other sites, including some of the rho-dependent and putative terminators.

Lambda red-dependent growth and recombination of phage P22

Plasmids that express lambda recombination functions singly and in combinations, at controllable levels, have been constructed. These plasmids were placed in Escherichia coli and Salmonella strains, and their ability to complement lambda and P22 strains lacking recombination functions was examined. The combination of lambda bet and exo constitutes a minimal system that can substitute for the recombination system of P22 in allowing efficient growth and recombination in a recA- host.

Genetics of bacteriophage phi 80--a review

The genetic maps of bacteriophage lambda and lambdoid phage phi 80 are compared. The gene organization of phi 80 is very similar to that of lambda, as shown by isolation and characterization of many am, ts and c (clear) mutants of the phage. In general, the essential genes located in the same position on the genetic map of the phages lambda and phi 80 fulfill the same functions. These include the gene clusters coding for the head and tail proteins, genes for DNA synthesis, and the genes controlling lysogeny and late gene expression. The specific regulatory features of phi 80 in relation to the N function of lambda are discussed, but they require further clarification. The two phages differ in immunity specificity, host range, conversion property and temperature sensitivity.

Recognition of messenger RNA during translational initiation in Escherichia coli

The structural aspects of recognition by E. coli ribosomes of translational initiation regions on homologous messenger RNAs have been reviewed. Also discussed is the location of initiation region on mRNA, its confines, typical nucleotide sequences responsible for initiation signal, and the influence of RNA macrostructure on protein synthesis initiation. Most of the published DNA nucleotide sequences surrounding the start of various E. coli genes and those of its phages have been collected.

Expression of the phage lambda recombination genes exo and bet under lacPO control on a multi-copy plasmid

The bacteriophage lambda genes exo and bet, whose products (lambda exonuclease and beta protein, respectively; Red phenotype) mediate homologous recombination of lambda phages, have been cloned under lacPO lacIq control on multi-copy plasmids. Induction of recA3 cells harboring these plasmids with isopropylthiogalactoside (IPTG) resulted in lambda exonuclease levels (assayed in vitro) that were proportional to the time of induction (for at least 4 h); recombination of lambda Red- phages in vivo was similarly inducible. Only one out of 25 bet delta plasmids (constructed by a variety of in vitro techniques) expressed lambda exonuclease, a result consistent with the polarity of several known phage bet mutations. A general method for transferring phage exo and bet mutations to plasmids was devised and plasmids bearing polar (bet3) and nonpolar (bet113) mutations were constructed. Mutant derivatives of the plasmid showed the same complementation pattern as analogous phage red mutants. When lambda bet3 phages (Exo-Bet-) infected IPTG-induced recA3 bacteria containing exo+bet+ plasmids, recombination frequencies were not more than twice those typical for infection of plasmid-free recA3 cells with exo+bet+ phages, even in the case of IPTG induction sufficient to elevate the production of lambda exonuclease about 100-fold. Even when plasmid induction was delayed till as late as 50 min after infection, recombination was significant. Preliminary experiments suggest that these plasmids encode a polypeptide with Gam activity that corresponds to the 98-amino acid "shorter" open reading frame assigned to gam by Sanger et al. (J. Mol. Biol. 162 (1982) 729-773).

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.

Stability of cloned promoter-containing fragments

Strong bacteriophages lambda and T7 promoters for Escherichia coli RNA polymerase were cloned in a multicopy plasmid. To achieve this result, two variants of the promoter-probe vectors were constructed. It was found that (i) modifications of the nucleotide sequence, apart from the commonly accepted promoter region, both upstream and downstream of the RNA initiation point greatly influenced the efficiency of promoters in vivo, (ii) a recombinant DNA composed of one of the promoter-probe plasmids and a tandem of A1, A2, and A3 promoters of T7 bacteriophage DNA induced a reproducible secondary change in plasmid DNA upon cloning. This change was substitution of the part of the recombinant that originated as T7 by a large portion of the host DNA.

Sequence changes in coliphage lambda mutants affecting the nutL antitermination site and termination by tL1 and tL2

The 17-bp sequence designated nutL is required for the N-mediated antitermination of transcription in the major leftward operon of coliphage lambda. The single-stranded sequence can be folded into a hairpin structure. Ten independently isolated spontaneous lambda nutL- mutants have changes that affect the same nucleotide, located in the loop of the hairpin structure, changing the guanine to adenine, thymine or cytosine. Another mutant (lambda nutL3), selected by a different means, has a deletion of one GC base pair and thus eliminates one C in the stem of the hairpin structure, destabilizing it -11.2 to -2.2 kcal/mol. True reversions of the nutL point mutations restore the guanine. The second-site revertant lambda ninL99 was found to have a deletion of 417 bp between the tL1 terminator and the N gene, removing bases +523 to +939 (counted from SL = +1). This deletion include codons for the six carboxy-terminal amino acids of gene N product, but the fusion allows continuation of translation for 53 additional amino acid residues beyond the truncated N gene before reaching a nonsense codon. The fused N product is active.

The rex region of bacteriophage lambda: two genes under three-way control

The nucleotide sequence of the phage lambda rex region consists of 1428 bp and codes for two genes, rexA and rexB. Hence the complete lambda immunity region codes for four genes and covers 2664 bp of sequence unique to lambda, as defined by the left and right boundaries of the imm434 region. Coordinate expression of both rexA and rexB, which are co-regulated with the cI repressor gene from promoters p rm and p re is responsible for the Rex phenotype, i.e. exclusion of a wide variety of superinfecting phage such as T4rII. The position of a third promoter, p lit, which overlaps the carboxy-terminal end of the rexA coding region, permits expression of rexB without rexA, from the resulting 470 nucleotide lit RNA. The lit transcript, therefore, must act as messenger for rexB in the noncoordinate expression of the rex genes that occurs late in lambda lytic infection. The coordinate and noncoordinate expression of rexB and rexA suggests a dual role for the very hydrophobic rexB protein. Studies of lambda early and late DNA replication implicate rexB as having auxiliary functions in both lysogenisation and lytic infection.

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.

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.

Antitermination and termination functions of the cloned nutL, N, and tL1 modules of coliphage lambda

A plasmid containing a pp-galK operon was constructed to assay galK expression as a measure of transcriptional regulation by the cloned nutL, N and tL1 modules in a Rho+, Nus+ Escherichia coli host. Insertion of tL1 and rut, both carried on a lambda fragment located between the bamHI site and gene N, between the promoter and galK reduced expression of galK by 80--90%. Gene N alone, when controlled by pp, stimulated galK expression by about two-fold. Cloning of both gene N and nutL in the proper orientation resulted in about a 60% decrease in the tL1 termination. Additional tandem nutL sites increased efficiency of antitermination. A shortened nutL segment, containing only 25 bp of the original genomic nutL sequence, was found to have nearly equal ability to bring about antitermination. If the orientation of the nutL fragment is reversed, antitermination is abolished and the insert now displays a termination function. Termination efficiency is 60 to 73% for one to four tandem nutL modules in reverse orientation. Similarly, the inverted N module acts as a terminator, with 67% efficiency for one and 90% for two tandem inserts. Termination by inverted nutL or N fragments is probably unrelated to their normal functions, but indicates a fortuitous presence of a terminator sequence in the inverted orientation.