Read-through transcription from a derepressed Tn3 promoter affects ColE1 functions on a ColE1::Tn3 composite plasmid

Influence of Tn917 insertion on transcription of the icaADBC operon in six biofilm-negative transposon mutants of Staphylococcus epidermidis

Insertion of Tn917 into the icaADBC operon determines a biofilm-negative phenotype in biofilm-producing Staphylococcus epidermidis due to the inactivation of the genes responsible for the synthesis of the polysaccharide intercellular adhesin. We previously characterized six isogenic biofilm-negative transposon mutants of S. epidermidis 1457 with Tn917 insertions in either icaA or icaC. Northern blot analysis using ica- and Tn917-specific probes revealed that ica sequences located upstream and downstream of the transposon insertion site were still transcribed in five mutants in which Tn917 was inserted in the same transcriptional orientation. Outward-directed transcription initiating from within the transposon resulted in the complete expression of individual ica genes. Our results indicate that not only the inactivation of the entire operon but the isolated interruption of individual icaA and icaC genes led to a biofilm-negative phenotype in S. epidermidis. Tn917 mutagenesis may also result in the activation of genes located downstream of the insertion site.

A novel insertion sequence (IS)-like element of the thermophilic bacterium PS3 promotes expression of the alanine carrier protein-encoding gene

A novel insertion sequence (IS)-like element was found in the 5'-upstream region of the alanine carrier protein-encoding gene (acp) in the thermophilic bacterium PS3 chromosomal DNA. The sequence contained an open reading frame (ORF) encoding a polypeptide of 369 amino acids which revealed high similarity with ORFs from IS891 from the cyanobacterium Anabaena and IS1136 from Saccharopolyspora erythraea. The direction of transcription was the same as that of acp, and typical inverted and direct repeats characteristic of IS were found in both the 5' and 3' region of the ORF. Southern hybridization analysis of the chromosomal DNA revealed that multiple copies of the ORF sequence were contained in the PS3 genome. This element might well be a member of a new IS family including IS891 and IS1136, and we have designated this element IS1341. The analysis of acp expression in Escherichia coli cells indicated that IS1341 promotes the expression of acp.

Transcriptional activation of flanking sequences by Tn1000 insertion

The carotenoid biosynthesis operon of Erwinia herbicola Eho13 consists of five genes, which are organized in the order crtE-crtX-crtY-crtI-crtB. These genes, with the exception of crtX, encode functions of beta-carotene biosynthesis and give an orange-coloured phenotype in Escherichia coli. Since crtX is not involved in the biosynthesis of beta-carotene, deletion of this gene does not alter the phenotype of pigmented cells. On the other hand, insertion of Tn1000 into crtX or into the upstream untranslated region of the operon resulted in a light-yellow, rather than an unpigmented phenotype, indicating that Tn1000 does not exert a strong polar effect when inserted in this operon. RNA analysis revealed that the sequence downstream from the insertion site was transcribed at a low level. Primer extension showed that the "-35"-like sequence in the terminal inverted repeats was not responsible for the transcription of the downstream sequence. Furthermore, primer extension and polymerase chain reaction (PCR) studies revealed that RNA transcribed from the promoters inside of Tn1000 was extended through the terminal inverted repeats into the adjacent sequences. In addition Tn1000, in either orientation, was able to generate fusion transcripts when placed upstream of a promoter-less tetracycline-resistance gene and resulted in cells resistant to the drug. These results showed that Tn1000 insertion transcriptionally activates the DNA sequences adjacent to the transposon.

Transposon mutagenesis by Tn4560 and applications with avermectin-producing Streptomyces avermitilis

The Tn3-like Streptomyces transposon Tn4560 was used to mutagenize Streptomyces avermitilis, the producer of anthelmintic avermectins and the cell growth inhibitor oligomycin. Tn4560 transposed in this strain from a temperature-sensitive plasmid to the chromosome and from the chromosome to a plasmid with an apparent frequency of about 10(-4) to 10(-3) at both 30 and 39 degrees C. Auxotrophic and antibiotic nonproducing mutations were, however, obtained only with cultures that were kept at 37 or 39 degrees C. About 0.1% of the transposon inserts obtained at 39 degrees C caused auxotrophy or abolished antibiotic production. The sites of insertion into the S. avermitilis chromosome were mapped. Chromosomal DNA fragments containing Tn4560 insertions in antibiotic production genes were cloned onto a Streptomyces plasmid with temperature-sensitive replication and used to transport transposon mutations to other strains, using homologous recombination. This technique was used to construct an avermectin production strain that no longer makes the toxic oligomycin.

The kil-kor regulon of broad-host-range plasmid RK2: nucleotide sequence, polypeptide product, and expression of regulatory gene korC

Broad-host-range plasmid RK2 encodes several kil operons (kilA, kilB, kilC, kilE) whose expression is potentially lethal to Escherichia coli host cells. The kil operons and the RK2 replication initiator gene (trfA) are coregulated by various combinations of kor genes (korA, korB, korC, korE). This regulatory network is called the kil-kor regulon. Presented here are studies on the structure, product, and expression of korC. Genetic mapping revealed the precise location of korC in a region near transposon Tn1. We determined the nucleotide sequence of this region and identified the korC structural gene by analysis of korC mutants. Sequence analysis predicts the korC product to be a polypeptide of 85 amino acids with a molecular mass of 9,150 daltons. The KorC polypeptide was identified in vivo by expressing wild-type and mutant korC alleles from a bacteriophage T7 RNA polymerase-dependent promoter. The predicted structure of KorC polypeptide has a net positive charge and a helix-turn-helix region similar to those of known DNA-binding proteins. These properties are consistent with the repressorlike function of KorC protein, and we discuss the evidence that KorA and KorC proteins act as corepressors in the control of the kilC and kilE operons. Finally, we show that korC is expressed from the bla promoters within the upstream transposon Tn1, suggesting that insertion of Tn1 interrupted a plasmid operon that may have originally included korC and kilC.

Colicin synthesis and cell death

Colicin E1 is a small plasmid, containing the cea gene for colicin, the most prominent product of the plasmid. Colicin is a 56-kilodalton bacteriocin which is especially toxic to Escherichia coli cells that do not contain the plasmid. Under normal growth conditions very low levels of the plasmid are produced as a result of cea gene repression by the host LexA protein. Conditions that lower the concentration of LexA protein result in elevated levels of colicin synthesis. The LexA protein concentration can be lowered by exposing the cells to DNA-damaging reagents such as UV light or mitomycin C. This is because DNA damage signals the host SOS response; the response leads to activation of the RecA protease which degrades the LexA protein. DNA-damaging reagents result in very high levels of colicin synthesis and subsequent death of plasmid-bearing cells. Elevated levels of colicin are also produced in mutants of E. coli that are deficient in LexA protein. We found that comparably high levels of colicin can be produced in such mutants in the absence of cell death. In lexA strains carrying a defective LexA repressor, colicin synthesis shows a strong temperature dependence. Ten to twenty times more colicin is synthesized at 42 degrees C. This sharp dependence of synthesis on temperature suggests that there are factors other than the LexA protein which regulate colicin synthesis.

Tn501 insertion mutagenesis in Pseudomonas aeruginosa PAO

Transposon insertion mutagenesis of the Pseudomonas aeruginosa PAO chromosome with Tn1 and Tn501 was carried out using a mutant plasmid of R68::Tn501 temperature-sensitive for replication and maintenance. This method consists of three steps. Firstly, the temperature-independent, drug-resistant clones were selected from the strain carrying this plasmid. In the temperature-independent clones, the plasmid was integrated into the chromosome by Tn1- or Tn501-mediated cointegrate formation. Secondly, such clones were cultivated at a permissive temperature to provoke the excision of the integrated plasmid from the chromosome. Excision occurred by the reciprocal recombination between the two copies of Tn1 or Tn501 flanking the integrated plasmid, leaving one Tn1 or Tn501 insertion on the chromosome. Thirdly, the excised plasmid was cured by cultivating these isolates at a non-permissive temperature without selection for the drug resistance. Using this method, we isolated 1 Tn1-induced and 43 Tn501-induced auxotrophic mutations in this organism. Genetic mapping allowed us to identify two new genes, pur-8001 and met-8003. The Tn501-induced auxotrophic mutations were distributed non-randomly among auxotrophic genes, and the reversion of the mutations by precise excision of the Tn501 insertion occurred very rarely.