Fine structure mapping of the tryptophan genes in Pseudomonas putida

DNA sequences and characterization of four early genes of the tryptophan pathway in Pseudomonas aeruginosa

Two pairs of related but easily distinguishable genes for the two subunits of anthranilate synthase have been identified in Pseudomonas aeruginosa. These were cloned, sequenced, inactivated in vitro by insertion of an antibiotic resistance cassette, and returned to the P. aeruginosa chromosome, replacing the wild-type gene. Gene replacement implicated only one of the pairs in tryptophan biosynthesis. This report describes the cloning and sequencing of the tryptophan-related gene pair, designated trpE and trpG, and presents experiments implicating their gene products in tryptophan production. DNA sequence analysis as well as growth and enzyme assays of insertionally inactivated strains indicated that trpG is the first gene in a three-gene operon that also includes trpD and trpC. Complementation of Trp auxotrophs by R-prime plasmids (T. Shinomiya, S. Shiga, and M. Kageyama, Mol. Gen. Genet., 189:382-389, 1983) has shown that a large cluster of pyocin R2 genes is flanked at one end by trpE and the other end by trpDC; the physical map that was obtained shows the distance between trpE and trpDC to be about 25 kilobases. Our restriction map of the trpE and trpGDC regions agrees with data presented by Shinomiya et al.

Evolutionary differences in chromosomal locations of four early genes of the tryptophan pathway in fluorescent pseudomonads: DNA sequences and characterization of Pseudomonas putida trpE and trpGDC

Pseudomonas putida possesses seven structural genes for enzymes of the tryptophan pathway. All but one, trpG, which encodes the small (beta) subunit of anthranilate synthase, have been mapped on the circular chromosome. This report describes the cloning and sequencing of P. putida trpE, trpG, trpD, and trpC. In P. putida and Pseudomonas aeruginosa, DNA sequence analysis as well as growth and enzyme assays of insertionally inactivated strains indicated that trpG is the first gene in a three-gene operon that also contains trpD and trpC. In P. putida, trpE is 2.2 kilobases upstream from the trpGDC cluster, whereas in P. aeruginosa, they are separated by at least 25 kilobases (T. Shinomiya, S. Shiga, and M. Kageyama, Mol. Gen. Genet., 189:382-389, 1983). The DNA sequence in P. putida shows an open reading frame on the opposite strand between trpE and trpGDC; this putative gene was not characterized. Evidence is also presented for sequence similarities in the 5' untranslated regions of trpE and trpGDC in both pseudomonads; the function of these regions is unknown, but it is possible that they play some role in regulation of these genes, since all the genes respond to repression by tryptophan. The sequences of the anthranilate synthase genes in the fluorescent pseudomonads resemble those of p-aminobenzoate synthase genes of the enteric bacteria more closely than the anthranilate synthase genes of those organisms; however, no requirement for p-aminobenzoate was found in the Pseudomonas mutants created in this study.

Omegon-Km: a transposable element designed for in vivo insertional mutagenesis and cloning of genes in gram-negative bacteria

To combine the features of the omega interposons with the advantages of in vivo transposition mutagenesis, we have constructed an artificial transposon, called Omegon-Km. The Omegon-Km transposon is carried on the plasmid pJFF350 which can be conjugally mobilized into a broad range of Gram-negative bacteria. Omegon-Km is flanked, in inverted orientation, by synthetic 28-bp repeats derived from the ends of IS1. In addition, each end of Omegon-Km has the very efficient transcription and translation terminators of the omega interposon. Internally, Omegon-Km carries the selectable kanamycin (Km)-neomycin resistance gene (alph A) which is expressed well in many Gram-negative bacteria. The IS1 transposition functions are located on the donor plasmid but external to Omegon-Km. Thus, insertions of Omegon-Km are very stable because they lack the capacity for further transposition. Omegon-Km mutagenesis is performed by conjugal transfer of pJFF350 from Escherichia coli into any Gram-negative recipient strain in which this plasmid is unable to replicate. Those cells which have had a transposition event are selected by their resistance to Km. Very high frequencies of Omegon-Km transposition were observed in Pseudomonas putida. Preliminary experiments with other Gram-negative soil and water bacteria (Rhizobium leguminosarum, Paracoccus denitrificans) yielded mutants at reasonable levels. The presence of an E. coli-specific origin of replication (ori) within Omegon-Km allows the rapid and easy cloning, in E. coli, of the nucleotide sequences flanking the site of the transposition event.

Cosmid cloning of five Zymomonas trp genes by complementation of Escherichia coli and Pseudomonas putida trp mutants

A library of Zymomonas mobilis genomic DNA was constructed in the broad-host-range cosmid pLAFR1. The library was mobilized into a variety of Escherichia coli and Pseudomonas putida trp mutants by using the helper plasmid pRK2013. Five Z. mobilis trp genes were identified by the ability to complement the trp mutants. The trpF, trpB, and trpA genes were on one cosmid, while the trpD and trpC genes were on two separate cosmids. The organization of the Z. mobilis trp genes seems to be similar to the organization found in Rhizobium spp., Acinetobacter calcoaceticus, and Pseudomonas acidovorans. The trpF, trpB, and trpA genes appeared to be linked, but they were not closely associated with trpD or trpC genes.

camR, a negative regulator locus of the cytochrome P-450cam hydroxylase operon

A 4.27-kilobase insert from a HindIII DNA library of Pseudomonas putida carrying the CAM plasmid allowed coordinate expression of genes camD and camC under control of camR, an upstream regulator. The camC gene specifies cytochrome P-450cam, and camD specifies the 5-exo-alcohol dehydrogenase. A 1.38-kilobase deletion from the insert results in the constitutive expression of genes camC and camD; transformation in trans restores the substrate control, indicating that camR is a negative regulator.

Genetic and physical analyses of Caulobacter crescentus trp genes

Caulobacter crescentus trp mutants were identified from a collection of auxotrophs. Precursor feeding experiments, accumulation studies, and complementation experiments resulted in the identification of six genes corresponding to trpA, trpB, trpC, trpD, trpE, and trpF. Genetic mapping experiments demonstrated that the trp genes were in two clusters, trpCDE and trpFBA, and a 5.4-kilobase restriction fragment from the C. crescentus chromosome was isolated that contained the trpFBA gene cluster. Complementation experiments with clones containing the 5.4-kilobase fragment indicated that trpF was expressed in Escherichia coli and that all three genes were expressed in Pseudomonas putida. This expression was lost in both organisms when the pBR322 tet gene promoter was inactivated, indicating that all three genes were transcribed in the same orientation from the tet promoter. Thus, the C. crescentus promoters do not seem to be expressed in E. coli or P. putida. Complementation of the C. crescentus trp mutants indicated that the tet promoter was not necessary for expression in C. crescentus and suggested that at least two native promoters were present for expression of the trpF, trpB, and trpA genes. Taken together, these results indicate that C. crescentus promoters may have structures that are significantly different from the promoters of other gram-negative species.

Chromosome transfer and R-prime plasmid formation mediated by plasmid pULB113 (RP4::mini-Mu) in Alcaligenes eutrophus CH34 and Pseudomonas fluorescens 6.2

Plasmid pULB113 (RP4::mini-Mu), which contains the mini-Mu transposon, promoted both homologous and heterologous gene transfer from Pseudomonas fluorescens 6.2 and Alcaligenes eutrophus CH34. Homologous gene transfer in P. fluorescens 6.2 and A. eutrophus CH34 occurred at a frequency of 10(-4) to 10(-5), and recombinants inherited unselected recessive markers, suggesting a process of chromosome mobilization. Loci involved in autotrophic growth were among those transferred in A. eutrophus. In heterospecific matings, markers were transferred from P. fluorescens to A. eutrophus, Salmonella typhimurium LT2, and Escherichia coli, from A. eutrophus to P. fluorescens, and from Erwinia carotovora subsp. chrysanthemi to A. eutrophus. Heterospecific matings resulted in the formation of R-prime plasmids at frequencies of 10(-7) to 10(-4) per transferred plasmid. When S. typhimurium was the recipient, we observed R-prime plasmids with both restriction-proficient and restriction-deficient strains, although restriction markedly affected the frequency of transfer of pULB113. R-prime plasmids were quite stable, but lost the transposed marker more easily in a rec+ background than in a recA background, suggesting excision of transposed material by reciprocal recombination between flanking copies of mini-Mu. R-prime plasmids could be transferred easily into different recipients and were used in complementation studies. PstI restriction digests of four R-prime plasmids carrying P. fluorescens 6.2 DNA showed a number of additional bands, suggesting that several genes were transposed together with the selected marker on the plasmid.

Biochemical genetics of tryptophan synthesis in Pseudomonas acidovorans

Sixty independent tryptophan auxotrophs of Pseudomonas acidovorans were isolated and characterized for nutritional response to intermediates of the pathway, accumulation of intermediates, and levels of tryptophan-synthetic enzymes. Mutants for each of the seven proteins catalyzing the five steps of tryptophan synthesis were obtained. Transductional analysis established three unlinked chromosomal regions: trpE, trpGDC, and trpFBA. The order of the genes within the two clusters was not determined. The levels and enzymatic activities of wild-type and mutant strains indicated that trpE and trpGDC were repressed by tryptophan. In contrast, trpFBA was not derepressed significantly by starvation for tryptophan. The trpG mutants had an additional requirement for p-aminobenzoate, which suggested that anthranilate synthase subunit II also served as glutamine-binding protein in the analogous reaction catalyzed by p-aminobenzoate synthase. In addition, trpD mutants revealed the ability of P. acidovorans to degrade anthranilate via the beta-ketoadipate pathway.

Dissociation of the NIC plasmid aggregate in Pseudomonas putida

The NIC plasmid on conjugal transfer from Pseudomonas convexa Pc 1 to Pseudomonas putida PpG 1 dissociates into an independent fertility factor T and a nontransmissible NIC structural gene plasmid.

Fertility factors in Pseudomonas putida: selection and properties of high-frequency transfer and chromosome donors

The octane plasmid (OCT) in Pseudomonas putida strains has been shown to be transferred at low frequency. However, bacteria which had newly received this plasmid showed a transient increase in donor ability. Using Octane+ P. putida as the donor, the transfer of most chromosomal markers was shown to be independent of OCT transfer, whereas the mobilization of the octanoate catabolism genes (octanoic and acetate) was dependent on OCT plasmid transfer. The presence of a fertility factor termed FPo has been postulated to explain these results. Strains carrying only this fertility factor have been obtained from strains carrying both OCT and FPo plasmids. Strains in which the OCT plasmid was transferred at high frequencies have also been isolated, and chromosome mobilization by OCT and FPo has been compared. A different gradient of transmission by OCT and FPo has been observed. It has also been shown that chromosome transfer by OCT was dependent on the bacterial recombination system, whereas the chromosome transfer by FPo was unaffected by the presence of a rec mutation in the donor strain.

F'-plasmid transfer from Escherichia coli to Pseudomonas fluorescens

Various F' plasmids of Escherichia coli K-12 could be transferred into mutants of the soil strain 6.2, classified herein as a Pseudomonas fluorescens biotype IV. This strain was previously found to receive Flac plasmid (N. Datta and R.W. Hedges, J. Gen Microbiol. 70:453-460, 1972). ilv, leu, met, arg, and his auxotrophs were complemented by plasmids carrying isofunctional genes; trp mutants were not complemented or were very poorly complemented. The frequency of transfer was 10(-5). Subsequent transfer into other P. fluorescens recipients was of the same order of magnitude. Some transconjugants were unable to act as donors, and these did not lose the received information if subcultured on nonselective media. Use of F' plasmids helped to discriminate metabolic blocks in P. fluorescens. In particular, metA, metB, and argH mutants were so distinguished. In addition, F131 plasmid carrying the his operon and a supD mutation could partially relieve the auxotrophy of thr, ilv, and metA13 mutants, suggesting functional expression of E. coli tRNA in P. fluorescens. In P. fluorescens metA Rifr mutants carrying the F110 plasmid, which carried the E. coli metA gene and the E. coli rifs allele, sensitivity to rifampin was found to be dominant at least temporarily over resistance. This suggests interaction of E. coli and P. fluorescens subunits of RNA polymerase. his mutations were also complemented by composite P plasmids containing the his-nif region of Klebsiella pneumoniae (plasmids FN68 and RP41). nif expression could be detected by acetylene reduction in some his+ transconjugants. The frequency of transfer of these P plasmids was 5 X 10(-4).

Evidence for transductional shortening of the plasmid obtained by recombination between the TOL catabolic plasmid and the R91 R plasmid

The previously isolated plasmid pND3, arising from recombination between the TOL catabolic plasmid and the R plasmid R91, was transduced by pf16 inPseudomonas putida. Apparent transductional shortening was evident in 25% of the transduced pND3 plasmids. Transductants were isolated which had segregated the antibiotic resistance marker, transfer ability and some of the catabolic functions of the parent plasmid.

Regulation of enzyme synthesis in the tryptophan pathway of Acinetobacter calcoaceticus

In Acinetobacter calcoaceticus the seven genes coding for the enzymes responsible for tryptophan synthesis map at three chromosomal locations. Two three-gene clusters, one (trpGDC) specifying the small subunit of anthranilate synthase, phosphoribosyl transferase, and indoleglycerol phosphate synthase and the other (trpFBA) specifying phosphoribosyl anthranilate isomerase and both tryptophan synthase subunits, are not linked to each other or to the trpE gene specifying the large anthranilate synthase subunit. When regulation of trp gene expression is studied in the wild type, only the level of the trpF gene product decreases upon addition of tryptophan to the medium. Tryptophan starvation of tryptophan auxotrophs, however, results in increased levels of all the tryptophan enzymes; this and additional evidence suggests that the expression of all the trp genes is subject to repression. The trpGDC genes are coordinately controlled, and the trpE gene is regulated in parallel with them. The trpFBA genes are controlled neither coordinately nor in parallel with the other trp genes, but respond proportionally when compared with each other. So far, two types of constitutive mutants have been found. The first class of mutants apparently occurs in the structural gene for a repressor protein; this repressor locus is unlinked to any of the biosynthetic trp genes and affects only the expression of trpE and the trpGDC cluster. The second class contains mutants closely linked to the trpGDC region; they overproduce only the gene products of this cluster.

Evidence for autogenous regulation of Pseudomonas putida tryptophan synthase

Studies of a trpA mutant constitutive for tryptophan synthase production support the hypothesis of autogenous regulation (R. F. Goldberger, 1974; A. R. Proctor and I. P. Crawford, 1975) of the Pseudomonas putida trpAB loci.

The uptake of glucose and gluconate by Pseudomonas putida

The uptake of glucose and gluconate is under inductive control in Pseudomonas putida. Glucose, gluconate, and 2-ketogluconate were each good nutritional inducers of these transport abilities. Glucose and gluconate uptake obeyed saturation kinetics: the apparent Km for glucose was 6 mM and that for gluconate was 0.5 mM. Therefore, transport of both substrates appears to be mediated by enzyme-like carriers. Glucose and gluconate are parallel inhibitors for their uptake9 Strains selected for their inability totransport glucose were found to be deficient in gluconate uptake. The reverse was alsotrue: mutations affecting gluconate entry also blocked the uptake of glucose. These results demonstrate that a common carrier is involved in the uptake of both glucose and gluconate by P. putida cells.

Autogenous regulation of the inducible tryptophan synthase of Pseudomonas putida

Mutants blocked before indole-3-glycerol phosphate formation in the tryptophan biosynthetic pathway of P. putida ("early-blocked" mutants) are unable to use indole as a source of tryptophan for growth on minimal medium. The uninduced level of tryptophan synthase [EC 4.2.1.20; L-serine hydro-lyase (adding indole)] in such mutants was thought to be responsible for this property. We have shown that levels of indole higher than those previously tested will support growth of these mutants. In addition, the growth rate of these mutants on a given indole concentration was shown to be proportional to the synthase level induced under the same conditions. This apparent induction of tryptophan synthase by indole in "early-blocked" mutants was shown to be caused by formation of the normal effector molecule, indole-3-glycerol-P, from indole. Secondary mutations occur in "early-blocked" trp strains, which enable them to grow on low concentrations of indole. One type of "indole-utilization" mutation occurs in the trpA gene, inactivating its product. Tryptophan synthase is readily induced by low concentrations of indole in these mutants, even though they are unable to convert indole to indole-3-glycerol-P. We propose that the alpha-chain of the synthase has an autogenous regulatory function, serving as the repressor or the indole-3-glycerol-P recognition component of the repressor of the trpAB operon (synthase alpha-and beta-chains). Our hypothesis holds that the trpA type of "indole-utilization" mutation alters the repressor (synthase alpha-chain) so that indole as well as indole-3-glycerol-P serves as an effector molecule for tryptophan synthase induction.

Fusion and compatibility of camphor and octane plasmids in Pseudomonas

The octane (OCT) plasmid in Pseudomonas putida derived from the omega-hydroxylase-carrying strain of Coon and coworkers is transferable to the camphor (CAM) plasmid-bearing strain by conjugation or by transduction. While the majority of the Cam (+)Oct(+) exconjugants segregate Cam(+) or Oct(+) cells, exconjugants with stable Cam (+)Oct(+) phenotype (CAM-OCT) can be detected at a low frequency. The transductants are all of the CAM-OCT phenotype. In the stable Cam (+)Oct(+) strains, the OCT plasmid resembles the CAM plasmid with respect to curing by mitomycin C, transfer in conjugation, and reaction to ts (temperature-sensitive) mutation specifically affecting CAM plasmid replication. Therefore, it is suggested that certain regions of homology exist between the CAM and OCT plasmids that enable them to recombine to form a single plasmid, and to overcome the incompatibility barrier that prevents their coexisting.

Transmissible plasmid coding for the degradation of benzoate and m-toluate in Pseudomonas arvilla mt-2

Pseudomonas arvillamt-2 (ATCC 23073) has been shown to harbour a transmissible plasmid which codes for the degradation of benzoate andm-toluate. Plasmid-borne genetic information codes for the conversion of these compounds to catechol then the assimilation of catechol via themetacleavage pathway.

Enzymes of the tryptophan pathway in three Bacillus species

The tryptophan synthetic pathway was characterized in three species of Bacillus, B. subtilis, B. pumilus, and B. alvei. They share the common features of a pathway which is subject to tryptophan repression, contains no unexpected complexes among the five enzymes, exhibits dissociable anthranilate synthase enzymes which do not require phosphoribosyl transferase for amidetransfer activity, contains separate indoleglycerol phosphate synthase and phosphoribosylanthranilate isomerase enzymes, and contains similar tryptophan synthetase multimers. In looking at these characteristics in detail however, differences among the three species became apparent, as, for example, in the complementation observed between the alpha and beta(2) components of tryptophan synthetase, and the dissociation patterns of the large and small components of anthranilate synthase. The results demonstrate some pitfalls in attempting to compare multimeric enzymes in crude extracts from different organisms.

Mapping of the tryptophan genes of Acinetobacter calcoaceticus by transformation

Auxotrophs of Acinetobacter calcoaceticus blocked in each reaction of the synthetic pathway from chorismic acid to tryptophan were obtained after N-methyl-N'-nitro-N-nitrosoguanidine mutagenesis. One novel class was found to be blocked in both anthranilate and p-aminobenzoate synthesis; these mutants (trpG) require p-aminobenzoate or folate as well as tryptophan (or anthranilate) for growth. The loci of six other auxotrophic classes requiring only tryptophan were defined by growth, accumulation, and enzymatic analysis where appropriate. The trp mutations map in three chromosomal locations. One group contains trpC and trpD (indoleglycerol phosphate synthetase and phosphoribosyl transferase) in addition to trpG mutations; this group is closely linked to a locus conferring a glutamate requirement. Another cluster contains trpA and trpB, coding for the two tryptophan synthetase (EC 4.2.1.20) subunits, along with trpF (phosphoribosylanthranilate isomerase); this group is weakly linked to a his marker. The trpE gene, coding for the large subunit of anthranilate synthetase, is unlinked to any of the above. This chromosomal distribution of the trp genes has not been observed in other organisms.

Genetic basis of the biodegradation of salicylate in Pseudomonas

The genetic basis of the biodegradation of salicylate in Pseudomonas putida R1 has been studied. This strain utilizes the meta pathway for oxidizing salicylate through formation of catechol and 2-hydroxymuconic semialdehyde. The enzymes of the meta pathway are induced by salicylate but not by catechol, and the genes specifying these enzymes are clustered. The gene cluster can be eliminated from some salicylate-positive cells by treatment with mitomycin C and appears to exist inside the cell as an extrachromosomal element. This extrachromosomal gene cluster, termed the SAL plasmid, can be transferred by conjugation from P. putida R1 to a variety of other Pseudomonas species.

Regulation of the tryptophan synthetic enzymes in Clostridium butyricum

Experiments concerned with the regulation of the tryptophan synthetic enzymes in anaerobes were carried out with a strain of Clostridium butyricum. Enzyme activities for four of the five synthetic reactions were readily detected in wild-type cells grown in minimal medium. The enzymes mediating reactions 3, 4, and 5 were derepressed 4- to 20-fold, and the data suggest that these enzymes are coordinately controlled in this anaerobe. The first enzyme of the pathway, anthranilate synthetase, could be derepressed approximately 90-fold under these conditions, suggesting that this enzyme is semicoordinately controlled. Mutants resistant to 5-methyl tryptophan were isolated, and two of these were selected for further analysis. Both mutants retained high constitutive levels of the tryptophan synthetic enzymes even in the presence of repressing concentrations of tryptophan. The anthranilate synthetase from one mutant was more sensitive to feedback inhibition by tryptophan than the enzyme from wild-type cells. The enzyme from the second mutant was comparatively resistant to feedback inhibition by tryptophan. Neither strain excreted tryptophan into the culture fluid. Tryptophan inhibits anthranilate synthetase from wild-type cells noncompetitively with respect to chorismate and uncompetitively with respect to glutamine. The Michaelis constants calculated for chorismate and glutamine are 7.6 x 10(-5)m and 6.7 x 10(-5)m, respectively. The molecular weights of the enzymes estimated by zonal centrifugation in sucrose and by gel filtration ranged from 24,000 to 89,000. With the possible exception of a tryptophan synthetase complex, there was no evidence for the existence of other enzyme aggregates. The data indicate that tryptophan synthesis is regulated by repression control of the relevant enzymes and by feedback inhibition of anthranilate synthetase. That this enzyme system more closely resembles that found in Bacillus than that found in enteric bacteria is discussed.

Pseudomonas putida tryptophan synthetase

The two protein components of Pseudomonas putida tryptophan synthetase have been purified to homogeneity. Although there is general similarity between the Pseudomonas enzyme and that of the enteric bacteria, many differences were found. Components from Escherichia coli and P. putida do not stimulate each other enzymatically, and the enzymes differ in their response to monovalent cations. Serine deamination occurs best with the intact enzyme of P. putida, not with the beta(2) subunit alone as in E. coli. The amino acid compositions of the alpha subunits differ appreciably. These findings extend earlier studies showing differences between enteric organisms and pseudomonads in the regulation and genetic organization of the enzymes of the tryptophan pathway.

Pseudomonas putida tryptophan synthetase: partial sequence of the subunit

There is 50% identity in the sequences of the first 50 residues of the alpha chains of Escherichia coli and Pseudomonas putida. No deletions or additions of residues are found in this region, except for the N-terminal methionine residue which is missing in the polypeptide isolated from P. putida. Most of the residues which differ are chemically dissimilar, and half of them are specified by codons which differ by more than a single base. The two residues known by mutational analysis to be essential for catalysis in E. coli are conserved in P. putida. The potential taxonomic usefulness of information of this sort is analyzed.

New regulatory mutation affecting some of the tryptophan genes in Pseudomonas putida

Three indole analogues, 5-methylindole, 5-fluoroindole, and 7-methylindole, and the tryptophan analogue 5-fluorotryptophan were found to inhibit the growth of wild-type Pseudomonas putida. Mutants resistant to these analogues were obtained. Some of the 5-fluoroindole- and 5-fluorotryptophan-resistant strains exhibit an abnormality in the regulation of certain trp genes. These strains excrete anthranilate when grown in minimal medium in the presence or absence of the inhibitor. In these strains, the trpA, B, and D gene products, the first, second, and fourth enzymes of the tryptophan pathway, are produced in 20-fold excess over the normal wild-type level. The other enzymes of the pathway are unaffected. Exogenous tryptophan is still able to repress the expression of the trpABD cluster somewhat. Similarity between the 5-fluoroindole- and 5-fluorotryptophan-resistant strains suggests that the former compound becomes effective through conversion to the latter. Repression and derepression experiments with two anthranilate-excreting, 5-fluoroindole-resistant strains showed coordinate variation of the affected enzymes. The locus conferring resistance and excretion is not linked by transduction to any of the trp genes.

Transduction and genetic homology between Pseudomonas species putida and aeruginosa

Genetic crosses occur by transduction between the species Pseudomonas aeruginosa and P. putida. The frequency relative to intraspecific transfer is reduced and varies among markers, suggesting that these genomes contain discrete regions of homology and nonhomology.

Defective phage and chromosome mobilization in Pseudomonas putida

The transfer of transducing phage DNA in association with the mandelate genetic region of Pseudomonas putida strain PRS1 (termed pfdm) has been achieved by growing together mandelate-positive PpG2 cells harboring pfdm as an extrachromosomal element and mandelate-deleted PpG1 strains. This transfer is analogous to sexual conjugation in the enterobacteria. The transfer of pfdm elements is always associated with chromosome mobilization and some rare recombinants acquire genetic donor ability. We have therefore concluded that the pfdm elements are responsible for initiation of chromosome mobilization in a manner not yet fully understood.

Enzymes of tryptophan biosynthesis in Serratia marcescens

In Serratia marcescens, the tryptophan biosynthetic enzymes were formed coordinately. A number of tryptophan auxotrophs showed single biochemical lesions; several mutants showed pleiotropic effects. Sucrose density gradient centrifugation revealed an unique pattern of migration of the tryptophan biosynthetic enzymes. The repression response of the Serratia enzymes to exogenous tryptophan was fivefold more sensitive than that found in Escherichia coli. When this information is contrasted with the available information on the other Enterobacteriaceae, one is compelled to conclude that S. marcescens enjoys a rather marked evolutionary divergence from the other enteric organisms.