Characterization of trbC, a new F plasmid tra operon gene that is essential to conjugative transfer

Orientia tsutsugamushi: comprehensive analysis of the mobilome of a highly fragmented and repetitive genome reveals the capacity for ongoing lateral gene transfer in an obligate intracellular bacterium

IMPORTANCE

Obligate intracellular bacteria, or those only capable of growth inside other living cells, have limited opportunities for horizontal gene transfer with other microbes due to their isolated replicative niche. The human pathogen Ot, an obligate intracellular bacterium causing scrub typhus, encodes an unusually high copy number of a ~40 gene mobile genetic element that typically facilitates genetic transfer across microbes. This proliferated element is heavily degraded in Ot and previously assumed to be inactive. Here, we conducted a detailed analysis of this element in eight Ot strains and discovered two strains with at least one intact copy. This implies that the element is still capable of moving across Ot populations and suggests that the genome of this bacterium may be even more dynamic than previously appreciated. Our work raises questions about intracellular microbial evolution and sounds an alarm for gene-based efforts focused on diagnosing and combatting scrub typhus.

Plasmids pick a bacterial partner before committing to conjugation

Bacterial conjugation was first described by Lederberg and Tatum in the 1940s following the discovery of the F plasmid. During conjugation a plasmid is transferred unidirectionally from one bacterium (the donor) to another (the recipient), in a contact-dependent manner. Conjugation has been regarded as a promiscuous mechanism of DNA transfer, with host range determined by the recipient downstream of plasmid transfer. However, recent data have shown that F-like plasmids, akin to tailed Caudovirales bacteriophages, can pick their host bacteria prior to transfer by expressing one of at least four structurally distinct isoforms of the outer membrane protein TraN, which has evolved to function as a highly sensitive sensor on the donor cell surface. The TraN sensor appears to pick bacterial hosts by binding compatible outer membrane proteins in the recipient. The TraN variants can be divided into specialist and generalist sensors, conferring narrow and broad plasmid host range, respectively. In this review we discuss recent advances in our understanding of the function of the TraN sensor at the donor-recipient interface, used by F-like plasmids to select bacterial hosts within polymicrobial communities prior to DNA transfer.

Swine growth promotion with antibiotics or alternatives can increase antibiotic resistance gene mobility potential

Even though the use of antibiotics for food-producing animals may contribute to the emergence of antimicrobial resistance, antibiotics are still used as growth promoters. Due to consumer and regulatory pressures, the use of alternatives to antibiotics as growth promoters is increasing, thus more information is needed on their capability to disseminate antimicrobial resistance compared to antibiotics. We investigated the impacts of carbadox (antibiotic), copper sulfate and zinc oxide (metals) and mushroom powder (natural product) on the pig fecal resistome and microbiome. Antibiotic resistance gene (ARG) and mobile genetic element (MGE) abundances were measured using a high-throughput qPCR array with 382 primer pairs. Bacterial community composition was determined by 16S rRNA gene sequencing. More ARGs co-occurred with MGEs in the growth promoter group samples than in the control group samples. Community composition could not be linked to resistome in the growth promoter group samples, indicating a potential decoupling of ARGs and phylogeny. Additionally, machine-learning methods aided in defining the community and resistome differences in response to treatments. Since increased ARG mobility potential was the primary response to the dietary additives used in this study, we suggest that ARG mobility should be considered when designing antimicrobial use policies and antimicrobial resistance surveillances.

Protein Dynamics in F-like Bacterial Conjugation

Efficient in silico development of novel antibiotics requires high-resolution, dynamic models of drug targets. As conjugation is considered the prominent contributor to the spread of antibiotic resistance genes, targeted drug design to disrupt vital components of conjugative systems has been proposed to lessen the proliferation of bacterial antibiotic resistance. Advancements in structural imaging techniques of large macromolecular complexes has accelerated the discovery of novel protein-protein interactions in bacterial type IV secretion systems (T4SS). The known structural information regarding the F-like T4SS components and complexes has been summarized in the following review, revealing a complex network of protein-protein interactions involving domains with varying degrees of disorder. Structural predictions were performed to provide insight on the dynamicity of proteins within the F plasmid conjugative system that lack structural information.

The interaction of TraW and TrbC is required to facilitate conjugation in F-like plasmids

Bacterial conjugation, such as that mediated by the E. coli F plasmid, is a main mechanism driving bacterial evolution. Two important proteins required for F-pilus assembly and DNA transfer proficiency are TraW and TrbC. As members of a larger complex, these proteins assemble into a type IV secretion system and are essential components of pore formation and mating pair stabilization between the donor and the recipient cells. In the current report, we demonstrate the physical interaction of TraW and TrbC, show that TraW preferentially interacts with the N-terminal domain of TrbC, and that this interaction is important in restoring conjugation in traW/trbC knockouts.

Molecular characterization of antibiotic resistance in cultivable multidrug-resistant bacteria from livestock manure

Diverse antibiotic-resistance genes (ARGs) are frequently reported to have high prevalence in veterinary manure samples due to extensive use of antibiotics in farm animals. However, the characteristics of the distribution and transmission of ARGs among bacteria, especially among different species of multiple antibiotic-resistant bacteria (MARB), have not been well explored. By applying high-throughput sequencing methods, our study uncovered a vast MARB reservoir in livestock manure. The genera Escherichia, Myroides, Acinetobacter, Proteus, Ignatzschineria, Alcaligenes, Providencia and Enterococcus were the predominant cultivable MARB, with compositions of 40.6%-85.7%. From chicken manure isolates, 33 MARB were selected for investigation of the molecular characteristics of antibiotic resistance. A total of 61 ARGs and 18 mobile genetic elements (MGEs) were investigated. We found that 47 ARGs were widely distributed among the 33 MARB isolates. Each isolate carried 27-36 genes responsible for resistance to eight classes of antibiotics frequently used in clinic or veterinary settings. ARGs to the six classes of antibiotics other than streptogramins and vancomycin were present in all 33 MARB isolates with a prevalence of 80%-100%. A total of 12 MGEs were widely distributed among the 33 MARB, with intI1, IS26, ISaba1, and ISEcp1 simultaneously present in 100% of isolates. In addition, 9 gene cassettes within integrons and ISCR1 were detected among MARB isolates encoding resistance to different antibiotic classes. This is the first report revealing the general co-presence of multiple ARGs, various MGEs and ARG cassettes in different species of individual MARB isolates in chicken manure. The results highlight a much higher risk of ARGs spreading through livestock manure to humans than we expected.

Comparative Genomics of the Conjugation Region of F-like Plasmids: Five Shades of F

The F plasmid is the foremost representative of a large group of conjugative plasmids, prevalent in Escherichia coli, and widely distributed among the Enterobacteriaceae. These plasmids are of clinical relevance, given their frequent association with virulence determinants, colicins, and antibiotic resistance genes. Originally defined by their sensitivity to certain male-specific phages, IncF plasmids share a conserved conjugative system and regulatory circuits. In order to determine whether the genetic architecture and regulation circuits are preserved among these plasmids, we analyzed the natural diversity of F-like plasmids. Using the relaxase as a phylogenetic marker, we identified 256 plasmids belonging to the IncF/ MOB_F12group, present as complete DNA sequences in the NCBI database. By comparative genomics, we identified five major groups of F-like plasmids. Each shows a particular operon structure and alternate regulatory systems. Results show that the IncF/MOB_F12 conjugation gene cluster conforms a diverse and ancient group, which evolved alternative regulatory schemes in its adaptation to different environments and bacterial hosts.

Comparison of proteins involved in pilus synthesis and mating pair stabilization from the related plasmids F and R100-1: insights into the mechanism of conjugation

F and R100-1 are closely related, derepressed, conjugative plasmids from the IncFI and IncFII incompatibility groups, respectively. Heteroduplex mapping and genetic analyses have revealed that the transfer regions are extremely similar between the two plasmids. Plasmid specificity can occur at the level of relaxosome formation, regulation, and surface exclusion between the two transfer systems. There are also differences in pilus serology, pilus-specific phage sensitivity, and requirements for OmpA and lipopolysaccharide components in the recipient cell. These phenotypic differences were exploited in this study to yield new information about the mechanism of pilus synthesis, mating pair stabilization, and surface and/or entry exclusion, which are collectively involved in mating pair formation (Mpf). The sequence of the remainder of the transfer region of R100-1 (trbA to traS) has been completed, and the complete sequence is compared to that of F. The differences between the two transfer regions include insertions and deletions, gene duplications, and mosaicism within genes, although the genes essential for Mpf are conserved in both plasmids. F+ cells carrying defined mutations in each of the Mpf genes were complemented with the homologous genes from R100-1. Our results indicate that the specificity in recipient cell recognition and entry exclusion are mediated by TraN and TraG, respectively, and not by the pilus.

The identification of exported proteins with gene fusions to invasin

Exported proteins are integral to understanding the biology of bacterial organisms. They have special significance in pathogenesis research because they can mediate critical interactions between pathogens and eukaryotic cell surfaces. Further, they frequently serve as targets for vaccines and diagnostic tests. The commonly used genetic assays for identifying exported proteins use fusions to alkaline phosphatase or beta-lactamase. These systems are not ideal for identifying outer membrane proteins because they identify a large number of inner membrane proteins as well. We addressed this problem by developing a gene fusion system that preferentially identifies proteins that contain cleavable signal sequences and are released from the inner membrane. This system selects fusions that restore outer membrane localization to an amino terminal-truncated Yersinia pseudotuberculosis invasin derivative. In the present study, a variety of Salmonella typhimurium proteins that localize beyond the inner membrane were identified with gene fusions to this invasin derivative. Previously undescribed proteins identified include ones that share homology with components of fimbrial operons, multiple drug resistance efflux pumps and a haemolysin. All of the positive clones analysed contain cleavable signal sequences. Moreover, over 40% of the genes identified encode putative outer membrane proteins. This system has several features that may make it especially useful in the study of genetically intractable organisms.

Role of the propilin leader peptide in the maturation of F pilin

F-pilin maturation and translocation result in the cleavage of a 51-amino-acid leader sequence from propilin and require LepB and TraQ but not the SecA-SecY secretion pathway. The unusual propilin leader peptide and the dependence of its cleavage on TraQ suggested that TraQ recognition may be specific for the leader peptide. An in vitro propilin cleavage assay yielded propilin (13 kDa), the pilin polypeptide (7 kDa), and a 5.5-kDa protein as the traA products. The 5.5-kDa protein comigrates with the full-length 51-amino-acid leader peptide, and [14C]proline labeling confirmed its identity since the only proline residues of propilin are found within the leader peptide. The in vitro and in vivo propilin-processing reactions proceed similarly in a single polypeptide cleavage step. Furthermore, TraQ dependence is a property of F-pilin maturation specifically rather than a property of the leader peptide. A propilin derivative with an amino-terminal signal sequence generated by deleting codons 2 to 28 required TraQ for processing in vivo. On the other hand, a chimeric protein with the propilin wild-type leader peptide fused to the mature portion of beta-lactamase was processed in a TraQ-independent manner. Thus, despite its unusual length, the propilin leader peptide seems to perform a function similar to that of the typical amino-terminal signal sequence. This work suggests that TraQ is not necessary for the proteolysis of propilin and therefore is likely to act as a chaperone-like protein that promotes the translocation of propilin.

Membrane insertion of the F-pilin subunit is Sec independent but requires leader peptidase B and the proton motive force

F pilin is the subunit required for the assembly of conjugative pili on the cell surface of Escherichia coli carrying the F plasmid. Maturation of the F-pilin precursor, propilin, involves three F plasmid transfer products: TraA, the propilin precursor; TraQ, which promotes efficient propilin processing; and TraX, which is required for acetylation of the amino terminus of the 7-kDa pilin polypeptide. The mature pilin begins at amino acid 52 of the TraA propilin sequence. We performed experiments to determine the involvement of host cell factors in propilin maturation. At the nonpermissive temperature in a LepBts (leader peptidase B) host, propilin processing was inhibited. Furthermore, under these conditions, only full-length precursor was observed, suggesting that LepB is responsible for the removal of the entire propilin leader peptide. Using propilin processing as a measure of propilin insertion into the plasma membrane, we found that inhibition or depletion of SecA and SecY does not affect propilin maturation. Addition of a general membrane perturbant such as ethanol also had no effect. However, dissipation of the proton motive force did cause a marked inhibition of propilin processing, indicating that membrane insertion requires this energy source. We propose that propilin insertion in the plasma membrane proceeds independently of the SecA-SecY secretion machinery but requires the proton motive force. These results present a model whereby propilin insertion leads to processing by leader peptidase B to generate the 7-kDa peptide, which is then acetylated in the presence of TraX.

Characterization of traX, the F plasmid locus required for acetylation of F-pilin subunits

Acetylation of F-pilin subunits has previously been shown to depend upon expression of the F plasmid transfer operon gene traX. To assess the requirement for pilin acetylation in conjugative transfer of F, we constructed traX::kan insertion mutations and crossed them onto the transmissible F derivative pOX38. Under standard conditions, the function of traX seemed to be dispensable. Although pilin synthesized by mutant plasmids pOX38-traX482 and pOX38-traX483 was not acetylated, F-pilus production and F-pilus-specific phage infection appeared to be normal and transfer occurred at wild-type frequency. Analysis of labeled products showed that TraX+ plasmids expressed two approximately 24- (TraX1) and 22-kDa (TraX2) polypeptides that localized in the cytoplasmic membranes of cells. No product that was similar in size to the product predicted from the traX open reading frame (27.5 kDa) was detected. Therefore, we used site-directed mutagenesis, stop codon linker insertions, and phoA fusion analysis to investigate traX expression. Both TraX1 and TraX2 appeared to be encoded by the traX open reading frame. Insertion of a stop codon linker into the traX C-terminal coding region led to synthesis of two correspondingly truncated products, and fusions to phoA indicated that only the traX reading frame was translated. Expression was also very dependent on the traX M1 start codon; when this was altered, no protein products were observed. However, pilin acetylation activity was still detectable, indicating that some other in-frame start codon(s) can also be used. All sequences that are essential for activity are contained between traX codons 29 and 225. Sequence analysis indicated that traX mRNA is capable of forming a variety of base-paired structures. We suggest that traX expression is translationally controlled and that F-pilin acetylation activity may be regulated by physiological conditions in cells.

Analysis of the sequence and gene products of the transfer region of the F sex factor

Bacterial conjugation results in the transfer of DNA of either plasmid or chromosomal origin between microorganisms. Transfer begins at a defined point in the DNA sequence, usually called the origin of transfer (oriT). The capacity of conjugative DNA transfer is a property of self-transmissible plasmids and conjugative transposons, which will mobilize other plasmids and DNA sequences that include a compatible oriT locus. This review will concentrate on the genes required for bacterial conjugation that are encoded within the transfer region (or regions) of conjugative plasmids. One of the best-defined conjugation systems is that of the F plasmid, which has been the paradigm for conjugation systems since it was discovered nearly 50 years ago. The F transfer region (over 33 kb) contains about 40 genes, arranged contiguously. These are involved in the synthesis of pili, extracellular filaments which establish contact between donor and recipient cells; mating-pair stabilization; prevention of mating between similar donor cells in a process termed surface exclusions; DNA nicking and transfer during conjugation; and the regulation of expression of these functions. This review is a compendium of the products and other features found in the F transfer region as well as a discussion of their role in conjugation. While the genetics of F transfer have been described extensively, the mechanism of conjugation has proved elusive, in large part because of the low levels of expression of the pilus and the numerous envelope components essential for F plasmid transfer. The advent of molecular genetic techniques has, however, resulted in considerable recent progress. This summary of the known properties of the F transfer region is provided in the hope that it will form a useful basis for future comparison with other conjugation systems.

The Escherichia coli K-12 F plasmid gene traX is required for acetylation of F pilin

The Escherichia coli F plasmid gene required for amino-terminal acetylation of F-pilin subunits was identified. Using Western blots (immunoblots), we assayed the reaction of monoclonal antibodies with F-pilin polypeptides in inner membrane preparations from various F mutant strains. It was known that JEL92 recognizes an internal pilin epitope and JEL93 recognizes the acetylated amino-terminal sequence (L.S. Frost, J.S. Lee, D.G. Scraba, and W. Paranchych, J. Bacteriol. 168:192-198, 1986). As expected, neither antibody reacted with inner membranes from F- cells or Flac derivatives that do not synthesize pilin. Mutations that affected the individual activities of F tra genes traA, -B, -C, -D, -E, -F, -G, -H, -I, -J, -K, -L, -M, -N, -P, -R, -U, -V and -W or trb genes trbA, -B, -C, -D, -E, -G, -H, and -I did not prevent JEL92 or JEL93 recognition of membrane pilin. However, Hfr deletion mutants that lacked the most-distal transfer region genes did not express pilin that reacted with JEL93. Nevertheless, all strains that retained traA and traQ did express JEL92-reactive pilin polypeptides. Analysis of strains expressing cloned tra segments showed that traA and traQ suffice for synthesis of JEL92-reactive pilin, but synthesis of JEL93-reactive pilin is additionally dependent on traX. We concluded that the traX product is required for acetylation of F pilin. Interestingly, our data also showed that TraA+ TraQ+ cells synthesize two forms of pilin which migrate at approximately 7 and 8 kDa. In TraX+ cells, both become acetylated and react with JEL93. Preparations of wild-type F-pilus filaments contain both types of subunits.

Construction and analysis of F plasmid traR, trbJ, and trbH mutants

F plasmid derivatives carrying kan insertion mutations in the transfer region genes traR, trbJ, and trbH were constructed. Standard tests indicated that these loci are not essential for F pilus production or F transfer among Escherichia coli K-12 hosts. Among the traR and trbH mutants tested, the orientation of the kan cassette had no effect on the mutant phenotype. In each case, there was no significant effect on the appearance of F pili, the transfer frequency, or the plating efficiency of F-pilus-specific phages. The trbJ insertion carrying a kan gene oriented in the direction opposite to tra transcription had very little effect on phage sensitivity but markedly reduced the plasmid transfer efficiency. However, the kan insertion mutation at the same site, in the tra orientation, did not seem to affect either property. Analysis of clones carrying trbJ sequences regulated by a phage T7 promoter showed that trbJ expresses an approximately 11-kDa protein product. The TrbJ protein was not expressed from clones carrying a kan insertion or stop codon linker insertion in the trbJ sequence. However, it was expressed from clones that did not include sequences at the beginning of the 113-codon open reading frame in this region. Our data indicated that translation of trbJ must be initiated at the more distal GUG codon in this frame. This would result in expression of a 93-amino-acid polypeptide.

Sequence alterations affecting F plasmid transfer gene expression: a conjugation system dependent on transcription by the RNA polymerase of phage T7

We constructed derivatives of the Escherichia coli conjugative plasmid F that carry altered sequences in place of the major transfer operon promoter, PY. Replacement of PY with a promoter-deficient sequence resulted in a transfer-deficient, F-pilus-specific phage-resistant plasmid (pOX38-tra701) that could still express TraJ and TraT; TraY, F-pilin, TraD, and TraI were not detectable on Western blots. On a second plasmid (pOX38-tra715) we replaced PY with a phage T7 late promoter sequence. In hosts carrying a lacUV5-promoter-regulated T7 RNA polymerase gene, all transfer-associated properties of pOX38-tra715 could be regulated with IPTG. After induction, pOX38-tra715 transferred at the wild-type frequency, expressed normal numbers of F-pili and conferred sensitivity to pilus-specific phages. No adverse effects on cell viability were apparent, and additional mutations could easily be crossed onto pOX38-tra715. A traJ deletion (pOX38-tra716) had no effect on the IPTG-induced transfer phenotype. Insertion of cam into trbC, resulted in a mutant (pOX38-tra715trbC33) which, after induction, exhibited the same phenotype associated with other trbC mutants; it could also be complemented by expression of trbC in trans. With pOX38-tra715 or its derivatives, we were able to label specifically the products of tra genes located throughout the long tra operon, by using rifampicin. This feature can be used to investigate transfer protein interactions and to follow changes in these proteins that are associated with conjugal mating events.

Characterization, localization, and sequence of F transfer region products: the pilus assembly gene product TraW and a new product, TrbI

The traW gene of the Escherichia coli K-12 sex factor, F, encodes one of the numerous proteins required for conjugative transfer of this plasmid. We have found that the nucleotide sequence of traW encodes a 210-amino-acid, 23,610-Da polypeptide with a characteristic amino-terminal signal peptide sequence; in DNA from the F lac traW546 amber mutant, the traW open reading frame is interrupted at codon 141. Studies of traW expression in maxicells in the presence and absence of ethanol demonstrate that the traW product does undergo signal sequence processing. Cell fractionation experiments additionally demonstrated that mature TraW is a periplasmic protein. Electron microscopy also showed that F lac traW546 hosts do not express F pili, confirming that TraW is required for F-pilus assembly. Our nucleotide sequence also revealed the existence of an additional gene, trbI, located between traC and traW. The trbI gene encodes a 128-amino-acid polypeptide which could be identified as a 14-kDa protein product. Fractionation experiments demonstrated that TrbI is an intrinsic inner-membrane protein. Hosts carrying the pOX38-trbI::kan insertion mutant plasmids that we constructed remained quite transfer proficient but exhibited increased resistance to F-pilus-specific phages. Mutant plasmids pOX38-trbI472 and pOX38-trbI473 expressed very long F pili, suggestive of a pilus retraction deficiency. Expression of an excess of TrbI in hosts carrying a wild-type pOX38 plasmid also caused F-pilus-specific phage resistance. The possibility that TrbI influences the kinetics of pilus outgrowth and/or retraction is discussed.

Localization of TraC, a protein involved in assembly of the F conjugative pilus

TraC is one of the proteins encoded by the F transfer region of the F conjugative plasmid which is required for the assembly of F pilin into the mature F pilus structure. Overproduction of this protein from the plasmid pKAS2, which carries only traC, resulted in the formation of inclusion bodies from which soluble TraC was purified. When small amounts of TraC were produced from pKAS2, the protein was localized to the cytoplasm by using anti-TraC antibodies. Similar analysis of a set of TraC-alkaline phosphatase fusion proteins localized all of these fusion proteins to the cytoplasm. However, when TraC was expressed from the F plasmid, much of it appeared associated with the bacterial membrane fraction. Under these conditions, TraC does not appear to be part of the tip of the F pilus, as neither anti-TraC antibodies nor purified TraC had any effect on the infection of F-containing bacteria by the filamentous bacteriophage f1. These data suggest that TraC is normally associated with the membrane through interactions with other proteins specified by the tra region. This interaction may be via the carboxyl-terminal region of the TraC protein, as a mutant TraC protein containing an Arg-Cys substitution at amino acid 811 exhibits an interaction with the membrane weaker than that of the wild-type protein in the presence of the other Tra proteins.

Characterization of the F plasmid mating aggregation gene traN and of a new F transfer region locus trbE

The product of the F plasmid transfer gene, traN, is thought to be required for the formation of stable mating aggregates during F-directed conjugation. By testing chimeric plasmids that express F transfer region segments for complementation of F lac traN mutant transfer, we mapped traN to the F transfer region between trbC and traF. Both protein and DNA sequence analysis determined the traN product to be a large, 66,000-Mr, polypeptide that undergoes signal sequence processing. The mature polypeptide was associated with outer membrane protein fractions, and a protease accessivity test confirmed that at least one portion of TraN is exposed on the cell surface. Our DNA sequence analysis also revealed that another gene, trbE, is located between traN and traF. The product of trbE was identified and shown to be a small, integral, inner membrane protein. The mating efficiency and pilus-specific phage susceptibility of a trbE::kan insertion mutant suggested that trbE is not essential for F transfer from Escherichia coli K-12 under standard mating conditions.

Construction and characterization of derivatives carrying insertion mutations in F plasmid transfer region genes, trbA, artA, traQ, and trbB

We devised a method for construction of insertion mutations in F plasmid tra region genes as a means of investigating the functions associated with previously uncharacterized loci. First, we constructed mutations in vitro, by insertion of a kanamycin resistance gene into a unique restriction site within a tra region fragment carried by a small, chimeric plasmid. Second, we crossed the insertion mutations, in vivo, onto a plasmid containing the complete F tra region sequence (either F lac, or pOX38, a Tra+ F plasmid derivative). Using this method, we obtained F lac mutant derivatives carrying KmR gene insertions in traQ, and a set of pOX38 mutant derivatives carrying a KmR gene insertion in trbA, artA, traQ, or trbB. Analysis of these derivatives showed that insertion of a kan gene at the NsiI site of traQ resulted in transfer deficiency, F-pilus-specific-phage resistance and an absence of detectable F-pilin subunit synthesis. Since the traQ mutants regained a wild-type phenotype when complemented with a traQ+ plasmid clone, we concluded that traQ expression is essential to transfer and F-pilus synthesis. However, pOX38 derivatives carrying kan gene inserts in genes trbA, artA, or trbB retained F-pilus-specific phage sensitivity and transferred at normal levels. Thus, these three gene products may not be essential for F-transfer from Escherichia coli K-12 under standard mating conditions.