R-factor mutant capable of specifying hypersynthesis of penicillinase

Gene expression per gene dose, a specific measure of gene expression in aquatic microorganisms

A method for the measurement of specific levels of gene expression that combines target mRNA and target DNA quantitation has been developed. The use of target gene dose as a normalizing factor for mRNA provides an alternative to 16S or 23S rRNA, which are unsuitable for use in the environment because of their presence in nontarget organisms. Both target mRNA and DNA are recovered from replicate samples and detected by using antisense and sense single-stranded RNA gene probes. For efficient mRNA recovery, the use of Millipore Durapore filters and multiple extractions was necessary. Quantitation was performed by radiometric detection by using a beta-scanner and comparison of the sample signal against target mRNA and DNA standard curves. This method enabled the measurement of expression of the catechol-2,3-dioxygenase gene (xylE) contained on the thermoregulated plasmid pLV1013 in a marine Vibrio strain in culture and in the environment. In studies of the relationship between mRNA levels and enzyme activities, the appearance of enzyme activity lagged behind xylE mRNA synthesis by an hour after temperature induction. This suggests that mRNA analysis is well suited for determining rapid regulation of microbial gene expression at the transcriptional level in water column microbial populations.

pMS76, a plasmid capable of amplification by treatment with chloramphenicol

pMS76 is a nonconjugative, 5.54-megadalton plasmid. This plasmid is present in Escherichia coli K12 cells at about 20 copies per chromosome. In addition, the pMS76 plasmid can be mobilized by conjugative plasmids and it shares a number of other properties with the amplifiable ColE1 plasmid, including the ability to amplify copy number in the presence of chloramphenicol. However, pMS76 is compatible with ColE1-like replicons, pBR313, and with other multicopy plasmids, RSF1030 and pACYC184. Also the replication of pMS76 is rifampicin sensitive and requires DNA polymerase 1.

A mutant defective in partitioning of composite plasmid Rms201

Escherichia coli harboring mutant plasmids defective in maintenance stability (from the conjugative plasmid Rms201) showed a wide distribution of ampicillin resistance levels, as well as increased frequency of plasmid loss from the cell. The amounts of covalently closed circular deoxyribonucleic acid of mutant plasmid Rms268 and parental plasmid Rms201 per chromosome were 5.3 and 6.1%, respectively. The beta-lactamase activities of strains W3630(Rms268) and W3630(Rms201) were 0.56 and 0.44 U/mg of protein, respectively. Frequency of plasmid loss from W3630(Rms268) was about 0.8 to 1.2% per cell generation, 100 times more than that of the wild-type strain. Ampicillin resistance levels of the colonies harboring the mutant plasmid showed a wide distribution, from low (100 micrograms/ml) to high (1,600 micrograms/ml). A miniplasmid (pMS268) with a mass of 7 X 10(6) daltons and encoding ampicillin resistance was isolated from Rms268. Frequency of pMS268 loss from W3630(pMS268) was about 0.8 to 1.9% per cell generation. W3630(pMS268) also showed a wide range of distribution in the levels of ampicillin resistance. These results indicated that the copies of Rms268 in E. coli did not segregate evenly between daughter cells at cell division and that the gene involved was located on the miniplasmid.

Isolation and characterization of a composite plasmid Rms201 mutant temperature sensitive for replication

A mutant temperature-sensitive for R-plasmid replication, Rms201ts14, was isolated from composite plasmid Rms201 after mutagenesis of P1 transducing lysate with 100 mM hydroxylamine for 40 h at 37 degrees C. When Escherichia coli ML1410(Rms201ts14)(+) was grown at temperatures between 40 and 42 degrees C in L broth, antibiotic-sensitive cells were segregated. When the incubation temperature of ML1410(Rms201ts14)(+) in L-broth was shifted to 42 from 30 degrees C, the increase in the number of antibiotic-resistant cells ceased 90 min after the temperature shift. However, the total number of cells continuously increased, and only 3% of the cells retained the plasmid at 5 h after the temperature shift to 42 degrees C. At 30 degrees C the amounts of covalently closed circular deoxyribonucleic acid per chromosome of Rms201ts14 and Rms201 were 3.8 and 6.3%, respectively. Incorporation of radioactive thymidine into the covalently closed circular deoxyribonucleic acid of Rms201ts14 did not take place at 42 degrees C, whereas radioactive thymidine was incorporated into the covalently closed circular deoxyribonucleic acid of Rms201 at a rate of 4%/chromosome even at 42 degrees C. The synthesis of plasmid covalently closed circular deoxyribonucleic acid in a cell harboring Rms201ts14 was almost completely blocked at 42 degrees C. These results indicated that the gene(s) responsible for plasmid deoxyribonucleic acid replication was affected in the mutant Rms201ts14. Temperature-sensitive miniplasmid pMSts214, which has a molecular weight of 5.3 x 10(6) and encodes ampicillin resistance, was isolated from Rms201ts14. Similarly, miniplasmid pMS201, which encodes single ampicillin resistance, was isolated from its parent, Rms201, and its molecular weight was 4.7 x 10(6). These results indicate that the gene(s) causing temperature sensitivity for replication of Rms201 resides on the miniplasmid.

Deletion mutants from R plasmids with increased copy number

Rms 201-12 and Rms 201-46 are R mutants with increased copy number, and are derived from a conjugative plasmid Rms 201 that encodes resistance to five drugs, ampicillin (Apc), tetracycline (Tc), chloramphenicol (Cm), streptomycin (Sm), and sulfonamides (Sa). The mutants expressed increased levels of resistance to Apc, Sm, and Cm, and a decreased level of resistance to Tc than those of the parent Rms 201 plasmid. When the Rms 201-12+ or Rms 201-46+ cells were inoculated onto plates containing a high concentration of Tc, colonies developed on the plate at a frequency of 10-3 to 10-4 after overnight incubation. The cells grown on the Tc plate carried a tet (Tc gene)-deleted R mutant besides tet-possessing Rms 201-12 or Rms 201-46, and we isolated the tet-deleted R mutant by purifying the R+ cells on drug-free plates. On the other hand, various deletion mutants possessing tet were isolated by prolonged culture of the cells. We have presented a circular gene order of Rms 201 by comparing the genetic markers of all deletion mutants derived from Rms 201, Rms 201-46, and Rms 201-12. The gene(s) regulating the copy number was closely linked to the rep gene. The gene(s) specifying entry exclusion was jointly lost with the tra region.

Integration of temperature-sensitive nonconjugative plasmid carrying kanamycin gene into conjugative R plasmids

A nonconjugative R plasmid, rMS3, whose molecular weight was 2.4 X 10(7) daltons, possessed a kanamycin resistance gene and was thermosensitive in its maintenance in Escherichia coli strains. We mobilized rMS3 with a conjugative R plasmid, R100 or T-tet, and obtained cointegrates carrying all the parental resistance markers. Various markers of the cointegrates were frequently deleted by P1 transduction and the deletion patterns among the different cointegrates were differed from each other. The cointegrates were thermoresistant, but the thermosensitive replicon could be segregated from the thermoresistant cointegrate by deletion. Some cointegrates between rMS3 and T-tet showed a derepressed state of transferability because of the integration of rMS3 and T-tet showed a derepressed state of transferability because of the integration of rMS3 into the regulator gene of the transfer loci. The genome size of the cointegrate so far tested was the sum of the sizes of the parental plasmids, indicating that the whole genome of rMS3 could integrate into various sites of the conjugative plasmids R100 and T-tet.

Control of replication and segregation of R plasmid Rts1

A mutant plasmid, pTW2, which was derived from the integrated Rst1 genome in the Escherichia coli chromosome, was studied as to its mode of replication at 30 degrees C. When Proteus mirabilis Pm17 harboring pTW2 was grown in broth at 30 degrees C, a considerable number of R- segregants (approximately 40%) were consistently observed. This indicates that pTW2 is unstable even at the permissive temperature for the replication of Rts1. The pTW2+ cells in a culture were heterogeneous with respect to the level of kanamycin resistance, ranging from 500 to 4,000 mug of the drug per ml. The amount of pTW2 deoxyribonucleic acid (DNA) relative to the Pm17 chromosomal DNA was about fivefold as large as that of Rts1 DNA in an exponentially growing culture. In addition, pTW2 in P. mirabilis continued to replicate after the chromosome had ceased to replicate, which was shown in the study of the inhibition of protein synthesis. Contrary to pTW2, the parent plasmid Rts1 is highly stable, and the relative percent Rts1 DNA is maintained at approximately 7% in any cultural conditions at a permissive temperature. These results suggest that copies of pTW2 may not segregate evenly into the host progeny upon cell division and that the replication of pTW2 does not coordinate with that of the chromosome. A remarkable instability of pTW2 as well as an increase in the relative percent pTW2 DNA was also shown when E. coli were used as the host cells. These results suggest the possibility that there is a gene or a gene cluster on the Rst1 genome responsible for the control of both replication and segregation of Rts1.

Association of a penicillin resistance gene with a tetracycline resistance plasmid (PTP-2) in Staphylococcus aureus

On transduction with a lysogenic strain of Straphylococcus aureus isolated from a clinical specimen and having tetracycline (TC)-penicillin (PC)-chloramphenicol (CP)-resistant plasmids, the three-drug-resistant strain was frequently obtained. By repeatedly transducing from this strain, a strain (TP-2) having stable resistance to TC and PC could be obtained. In transformation with the deoxyribonucleic acid (DNA) of TP-2 as donor, all of the transformants obtained by selecting with either TC or PC were both TC and PC resistant. According to electron microscopy study of the covalently closed circular DNA of TP-2, the plasmid DNA size was 1.37 +/- 0.03 mum (2.84 x 10(6) daltons). The plasmid (P(TP-2)) is presumed to be a new plasmid in which the PC resistance gene was integrated into the TC-resistant plasmid.