Competition for nutritional resources masks the true frequency of bacterial mutants.

BACKGROUND: It is widely assumed that all mutant microorganisms present in a culture are able to grow and form colonies, provided that they express the features required for selection. Unlike wild-type Escherichia coli, PHO-constitutive mutants overexpress alkaline phosphatase and hence can hydrolyze glycerol-2-phosphate (G2P) to glycerol and form colonies on plates having G2P as the sole carbon source. These mutations mostly occur in the pst operon. However, the frequency of PHO-constitutive colonies on the G2P selective plate is exceptionally low. RESULTS: We show that the rate in which spontaneous PHO-constitutive mutations emerge is about 8.0 × 10-6/generation, a relatively high rate, but the growth of most existing mutants is inhibited by their neighboring wild-type cells. This inhibition is elicited only by non-mutant viable bacteria that can take up and metabolize glycerol formed by the mutants. Evidence indicates that the few mutants that do form colonies derive from microclusters of mutants on the selective plate. A mathematical model that describes the fate of the wild-type and mutant populations under these circumstances supports these results. CONCLUSION: This scenario in which neither the wild-type nor the majority of the mutants are able to grow resembles an unavoidable "tragedy of the commons" case which results in the collapse of the majority of the population. Cooperation between rare adjacent mutants enables them to overcome the competition and eventually form mutant colonies. The inhibition of PHO-constitutive mutants provides an example of mutant frequency masked by orders of magnitude due to a competition between mutants and their ancestral wild-type cells. Similar "tragedy of the commons-like" cases may occur in other settings and should be taken into consideration while estimating true mutant frequencies and mutation rates.

HK022 bacteriophage Integrase mediated RMCE as a potential tool for human gene therapy.

HK022 coliphage site-specific recombinase Integrase (Int) can catalyze integrative site-specific recombination and recombinase-mediated cassette exchange (RMCE) reactions in mammalian cell cultures. Owing to the promiscuity of the 7 bp overlap sequence in its att sites, active 'attB' sites flanking human deleterious mutations were previously identified that may serve as substrates for RMCE reactions for future potential gene therapy. However, the wild type Int proved inefficient in catalyzing such RMCE reactions. To address this low efficiency, variants of Int were constructed and examined by integrative site-specific recombination and RMCE assays in human cells using native 'attB' sites. As a proof of concept, various Int derivatives have demonstrated successful RMCE reactions using a pair of native 'attB' sites that were inserted as a substrate into the human genome. Moreover, successful RMCE reactions were demonstrated in native locations of the human CTNS and DMD genes whose mutations are responsible for Cystinosis and Duchene Muscular Dystrophy diseases, respectively. This work provides a steppingstone for potential downstream therapeutic applications.

Anti-cancer binary system activated by bacteriophage HK022 integrase.

The binary system presented in this work is based on the bacteriophage HK022 integrase recombinase that activates the expression of a silenced Diphtheria toxin gene, both controlled by the cancer specific hTERT promoter. Using a lung cancer mice model, assays of different apoptotic and anti-apoptotic factors have demonstrated that the Integrase based binary system is highly specific towards cancer cells and more efficient compared to the conventional mono system whose toxin is directly expressed under hTERT. In a mice survival test, this binary system demonstrated longer persistence compared to the untreated and the mono treated ones. The reason underlying the advantage of this binary system over the mono system seems to be an overexpression of various hTERT suppressing factors induced by the mono system.

Cancer-specific binary expression system activated in mice by bacteriophage HK022 Integrase.

Binary systems based on site-specific recombination have been used for tumor specific transcription targeting of suicide genes in animal models. In these binary systems a site specific recombinase or integrase that is expressed from a tumor specific promoter drives tumor specific expression of a cytotoxic gene. In the present study we developed a new cancer specific binary expression system activated by the Integrase (Int) of the lambdoid phage HK022. We demonstrate the validity of this system by the specific expression of a luciferase (luc) reporter in human embryonic kidney 293T (HEK293T) cells and in a lung cancer mouse model. Due to the absence viral vectors and of cytotoxicity the Int based binary system offers advantages over previously described counterparts and may therefore be developed into a safer cancer cell killing system.

Ugp and PitA participate in the selection of PHO-constitutive mutants.

UNLABELLED: Mutations that cause the constitutive expression of the PHO regulon of Escherichia coli occur either in the pst operon or in the phoR gene, which encode, respectively, a high-affinity Pi transport system and a histidine kinase sensor protein. These mutations are normally selected on glycerol-2-phosphate (G2P) as the carbon source in the presence of excess Pi. The emergence of early PHO-constitutive mutants, which appear after growth for up to 48 h on selective medium, depends on the presence of phoA, which codes for a periplasmic alkaline phosphatase, while late mutants, which appear after 48 h, depend both on phoA and on the ugp operon, which encodes a glycerophosphodiester transport system. The emergence of the late mutants hints at an adaptive mutation process. PHO-constitutive phoR mutants appear only in a host that is mutated in pitA, which encodes an alternative Pi transport system that does not belong to the PHO regulon. The conserved Thr(217) residue in the PhoR protein is essential for PHO repression. IMPORTANCE: One of the principal ways in which bacteria adapt to new nutrient sources is by acquiring mutations in key regulatory genes. The inability of E. coli to grow on G2P as a carbon source is used to select mutations that derepress the PHO regulon, a system of genes involved in the uptake of phosphorus-containing molecules. Mutations in the pst operon or in phoR result in the constitutive expression of the entire PHO regulon, including alkaline phosphatase, which hydrolyzes G2P. Here we demonstrate that the ugp operon, another member of the PHO regulon, is important for the selection of PHO-constitutive mutants under prolonged nutritional stress and that phoR mutations can be selected only in bacteria lacking pitA, which encodes a secondary Pi transport system.

Expression and functional characterization of Smyd1a in myofibril organization of skeletal muscles.

BACKGROUND: Smyd1, the founding member of the Smyd family including Smyd-1, 2, 3, 4 and 5, is a SET and MYND domain containing protein that plays a key role in myofibril assembly in skeletal and cardiac muscles. Bioinformatic analysis revealed that zebrafish genome contains two highly related smyd1 genes, smyd1a and smyd1b. Although Smyd1b function is well characterized in skeletal and cardiac muscles, the function of Smyd1a is, however, unknown. METHODOLOGY/PRINCIPAL FINDINGS: To investigate the function of Smyd1a in muscle development, we isolated smyd1a from zebrafish, and characterized its expression and function during muscle development via gene knockdown and transgenic expression approaches. The results showed that smyd1a was strongly expressed in skeletal muscles of zebrafish embryos. Functional analysis revealed that knockdown of smyd1a alone had no significant effect on myofibril assembly in zebrafish skeletal muscles. However, knockdown of smyd1a and smyd1b together resulted in a complete disruption of myofibril organization in skeletal muscles, a phenotype stronger than knockdown of smyd1a or smyd1b alone. Moreover, ectopic expression of zebrafish smyd1a or mouse Smyd1 transgene could rescue the myofibril defects from the smyd1b knockdown in zebrafish embryos. CONCLUSION/SIGNIFICANCE: Collectively, these data indicate that Smyd1a and Smyd1b share similar biological activity in myofibril assembly in zebrafish embryos. However, Smyd1b appears to play a major role in this process.

Efficient Flp-Int HK022 dual RMCE in mammalian cells.

Recombinase-mediated cassette exchange, or RMCE, is a clean approach of gene delivery into a desired chromosomal location, as it is able to insert only the required sequences, leaving behind the unwanted ones. RMCE can be mediated by a single site-specific DNA recombinase or by two recombinases with different target specificities (dual RMCE). Recently, using the Flp-Cre recombinase pair, dual RMCE proved to be efficient, provided the relative ratio of the enzymes during the reaction is optimal. In the present report, we analyzed how the efficiency of dual RMCE mediated by the Flp-Int (HK022) pair depends on the variable input of the recombinases-the amount of the recombinase expression vectors added at transfection-and on the order of the addition of these vectors: sequential or simultaneous. We found that both in the sequential and the simultaneous modes, the efficiency of dual RMCE was critically dependent on the absolute and the relative concentrations of the Flp and Int expression vectors. Under optimal conditions, the efficiency of 'simultaneous' dual RMCE reached ∼12% of the transfected cells. Our results underline the importance of fine-tuning the reaction conditions for achieving the highest levels of dual RMCE.

Arm site independence of coliphage HK022 integrase in human cells.

The integrase encoded by the lambdoid phage HK022 (Int-HK022) resembles its coliphage λ counterpart (Int-λ) in the roles of the cognate DNA arm binding sites and in controlling the direction of the reaction. We show here that within mammalian cells, Int-HK022 does not exhibit such a control. Rather, Int-HK022 recombined between all ten possible pairwise att site combinations, including attB × attB that was more effective than the conventional integrative attP × attB reaction. We further show that Int-HK022 depends on the accessory integration host factor (IHF) protein considerably less than Int-λ and exhibits stronger binding affinity to the att core. These differences explain why wild-type Int-HK022 is active in mammalian cells whereas Int-λ is active there only as an IHF-independent mutant.

Alternative promoters in the pst operon of Escherichia coli.

The pst operon of Escherichia coli is composed of five genes pstS, pstC, pstA, pstB and phoU, that encode a high-affinity phosphate transport system and a negative regulator of the PHO regulon. Transcription of pst is induced under phosphate shortage and is initiated at the promoter located upstream of the first gene of the operon, pstS. Here, we show by four different technical approaches the existence of additional internal promoters upstream of pstC, pstB and phoU. These promoters are not induced by Pi-limitation and do not possess PHO-box sequences. Plasmids carrying the pst internal genes partially complement chromosomal mutations in their corresponding genes, indicating that they are translated into functional proteins.

High efficiency of a sequential recombinase-mediated cassette exchange reaction in Escherichia coli.

A comparison between the efficiency of recombinase-mediated cassette exchange (RMCE) reactions catalyzed in Escherichia coli by the site-specific recombinases Flp of yeast and Int of coliphage HK022 has revealed that an Flp-catalyzed RMCE reaction is more efficient than an Int-HK022 catalyzed reaction. In contrast, an RMCE reaction with 1 pair of frt sites and 1 pair of att sites catalyzed in the presence of both recombinases is very inefficient. However, the same reaction catalyzed by each recombinase individually supplied in a sequential order is very efficient, regardless of the order. Atomic force microscopy images of Flp with its DNA substrates show that only 1 pair of recombination sites forms a synaptic complex with the recombinase. The results suggest that the RMCE reaction is sequential.

Transgenic organisms expressing genes from Bacillus thuringiensis to combat insect pests.

Various subspecies (ssp.) of Bacillus thuringiensis (Bt) are considered the best agents known so far to control insects, being highly specific and safe, easily mass produced and with long shelf life.1 The para-crystalline body that is produced during sporulation in the exosporium includes polypeptides named δ-endotoxins, each killing a specific set of insects. The different entomopathogenic toxins of various Bt ssp. can be manipulated genetically in an educated way to construct more efficient transgenic bacteria or plants that express combinations of toxin genes to control pests.2 Joint research projects in our respective laboratories during the last decade demonstrate what can be done by implementing certain ideas using molecular biology with Bt ssp. israelensis (Bti) as a model system. Here, we describe our progress achieved with Gram-negative bacterial species, including cyanobacteria, and some preliminary experiments to form transgenic plants, mainly to control mosquitoes (Diptera), but also a particular Lepidopteran and Coleopteran pest species. In addition, a system is described by which environment-damaging genes can be removed from the recombinants thus alleviating procedures for obtaining permits to release them in nature.

Site-specific recombination in the cyanobacterium Anabaena sp. strain PCC 7120 catalyzed by the integrase of coliphage HK022.

The integrase (Int) of the lambda-like coliphage HK022 catalyzes the site-specific integration and excision of the phage DNA into and from the chromosome of its host, Escherichia coli. Int recognizes two different pairs of recombining sites attP x attB and attL x attR for integration and excision, respectively. This system was adapted to the cyanobacterium Anabaena sp. strain PCC 7120 as a potential tool for site-specific gene manipulations in the cyanobacterium. Two plasmids were consecutively cointroduced by conjugation into Anabaena cells, one plasmid that expresses HK022 Int recombinase and the other plasmid that carries the excision substrate P(glnA)-attL-T1/T2-attR-lacZ, where T1/T2 are the strong transcription terminators of rrnB, to prevent expression of the lacZ reporter under the constitutive promoter P(glnA). The Int-catalyzed site-specific recombination reaction was monitored by the expression of lacZ emanating as a result of T1/T2 excision. Int catalyzed the site-specific excision reaction in Anabaena cells when its substrate was located either on the plasmid or on the chromosome with no need to supply an accessory protein, such as integration host factor and excisionase (Xis), which are indispensable for this reaction in its host, E. coli.

Optimization of coliphage HK022 Integrase activity in human cells.

The Integrase (Int) site-specific recombinase of coliphage HK022 catalyzes integrative and excisive DNA recombination between two attachment (att) sites in human cells without the need to supply the accessory proteins Integration Host Factor (IHF) and Excisionase (Xis). Previous work has shown that under these conditions, reactions in cis, i.e. both att sites are located on the same chromosome, can be detected without selection. However, recombination in trans, i.e. one att site positioned on a chromosome and the other on an episomal vector, was detected only after selection. Here we show that optimization of the int-HK022 gene for human codon usage according to the GeneOptimizer software algorithm, as well as addition of accessory proteins IHF and Xis improve the recombination efficiencies in human cells, such that recombinants in a trans reaction could be detected without selection.

Molecular analysis of recombinase-mediated cassette exchange reactions catalyzed by integrase of coliphage HK022.

The integrase (Int) protein of coliphage HK022 can catalyze in Escherichia coli as well as in in vitro integrative and excisive recombinase-mediated cassette exchange reactions between plasmids as substrates. Atomic force microscopy images have revealed that in the protein-DNA complexes that are formed, the plasmid substrates are connected via one and not two pairs of attachment sites. This observation, together with the elucidation of intermediate co-integrates between the two circular plasmids, suggest that a sequential mechanism of the RMCE reaction is possible.

Phosphorylation of the integrase protein of coliphage HK022.

The integrase (Int) proteins of coliphages HK022 and lambda, are phosphorylated in one or more of their tyrosine residues. In Int of HK022 the phosphorylated residue(s) belong to its core-binding/catalytic domains. Wzc, a protein tyrosine kinase of Escherichia coli, is not required for Int phosphorylation in vivo, however, it can transphosphorylate the conserved Tyr(342) catalytic residue of Int in vitro. Int purified from cells that overexpress Wzc has a reduced activity in vitro. In vivo, the lysogenization of wild type HK022 as well as of lambda is not affected by the overexpression of Wzc. However, the nin5 mutant of lambda, which lacks a protein-tyrosine phosphatase gene, shows a significantly reduced lysogenization. It is suggested that phosphorylation of Int by Wzc down regulates the activity of Int.

Human genomic site-specific recombination catalyzed by coliphage HK022 integrase.

It has been previously demonstrated that the wild type integrase (Int) protein of coliphage HK022 can catalyze site-specific recombination in human cells between attachment (att) sites that were placed on extrachromosomal plasmids. In the present report it is shown that Int can catalyze the site-specific recombination reactions in a human cell culture on the chromosomal level. These include integrative (attP x attB) as well as excisive (attL x attR) reactions each in two configurations. In the cis configuration both sites are on the same chromosome, in the trans configuration one site is on a chromosome and the other on an episome. The reactions in cis were observed without any selection force, using the green fluorescent protein (GFP) as a reporter. The reactions in trans could be detected only when a selection force was applied, using the hygromycin-resistant (Hyg(R)) phenotype as a selective marker. All reactions were catalyzed without the need to supply any of the accessory proteins that are required by Int in its Escherichia coli host. The versatility of the att sites may be an advantage in the utilization of Int to integrate plasmid DNA into the genome, followed by a partial exclusion of the integrated plasmid.

Specific electrochemical phage sensing for Bacillus cereus and Mycobacterium smegmatis.

The rapid and reliable detection of pathogenic microorganisms is an important issue for the safety and security of our society. Here we describe the use of a sensitive, inexpensive, amperometric, phage-based biosensor for the detection of extremely low concentrations of Bacillus cereus and Mycobacterium smegmatis as models for Bacillus anthracis (the causative agent of anthrax) and for Mycobacterium tuberculosis (the causative agent of tuberculosis), respectively. The detection procedure developed here enabled the determination of bacteria at a low concentration of 10 viable cells/mL within 8 h. This experimental setup allows the simultaneous analysis of up to eight independent samples, using disposable screen-printed electrodes.

The effect of IHF on sigmaS selectivity of the phoA and pst promoters of Escherichia coli.

The pst operon, a member of the PHO regulon of Escherichia coli, encodes a high-affinity phosphate transport system whose expression is induced when the cells enter a phase of phosphate starvation. The expression of pst is stimulated by the integration host factor (IHF). Transcription of the PHO regulon genes is initiated by the RNA polymerase complexed with sigma (D) (Esigma (D)). Owing to a cytosine residue at position -13 of the pst promoter its transcription can also be initiated by Esigma (S). Here, we show that inactivation of IHF in vivo abolishes the sigma (S)-dependent transcription initiation of the pst operon, indicating that both -13C residue and IHF are required to confer on pst the ability to be transcribed by Esigma (S). Introduction of a -13C residue in the promoter region of phoA, another PHO regulon gene that is not directly affected by IHF, did not affect its exclusive transcription initiation by Esigma (D).

Site-specific recombination in Arabidopsis plants promoted by the Integrase protein of coliphage HK022.

The gene encoding the wild type Integrase protein of coliphage HK022 was integrated chromosomally and expressed in Arabidopsis thaliana plants. Double-transgenic plants cloned with the int gene as well as with a T-DNA fragment carrying the proper att sites in a tandem orientation showed that Int catalyzed a site-specific integration reaction (attP x attB) as well as a site-specific excision reaction (attL x attR). The reactions took place without the need to provide any of the accessory proteins that are required by Int in the bacterial host. When expressed in tobacco plants a GFP-Int fusion exhibits a predominant nuclear localization.

Protein-protein interaction between monomers of coliphage HK022 excisionase.

Excisionase (Xis) is an accessory protein that is required for the site-specific excision reaction of the coliphages HK022 and lambda. Xis binds in a strong cooperative manner to two tandem binding sites (X1 and X2) located on the P arm of the attachment (att) sites on the phage genome. As a result of crosslinking experiments in vivo and in vitro of Xis-overexpressing cells, by gel filtration of purified Xis and by FRET analyses we show that Xis monomers of HK022 interact and form dimers that are not dependent on the single Cys residue of the protein and on the presence of DNA. The formation of the dimers may explain the strong binding cooperativity of Xis to its sites on DNA.