Relationships between autonomous and integrated forms of tetracycline resistance plasmid in Staphylococcus aureus

Nucleotide sequence and genetic characterization of staphylococcal bacteriophage L54a int and xis genes

The nucleotide sequence of a staphylococcal bacteriophage L54a DNA fragment containing genes involved in site-specific recombination was determined. Mutations generated by in vitro mutagenesis were used to map and characterize the int and xis genes. The site-specific recombination functions are tightly clustered within a 1.75-kilobase stretch of DNA fragment with the gene order of attP-int-xis. The int and xis genes are transcribed divergently. The Int protein deduced from the nucleotide sequence has a molecular weight of 41,000. Int is a basic protein with 354 amino acids of which 72 are basic and 38 are acidic. The Xis protein consists of only 59 amino acids with a molecular weight of 7,180. Unlike the Xis proteins of the lambdoid bacteriophages which are all basic proteins, L54a Xis is an acidic protein containing 13 acidic and 8 basic amino acids. The Int protein is required in both integrative and excisive reactions, whereas Xis is only required in excisive reaction. A well-conserved 40-residue region, including three perfectly conserved residues found in 15 site-specific recombinases of the integrase family that have been characterized, was also found in the L54a Int protein.

A site-specific recombination function in Staphylococcus aureus plasmids

All known small staphylococcal plasmids possess one or two recombination sites at which site-specific cointegrate formation occurs. One of these sites, RSA, is present on two small multicopy plasmids, pT181 and pE194; it consists of 24 base pairs of identity in the two plasmids, the "core," flanked by some 50 base pairs of decreasing homology. Here we show that recombination at RSA is recA independent and is mediated by a plasmid-encoded, trans-acting protein, Pre (plasmid recombination). Pre-mediated recombination is site specific in that it occurs within the core sequence of RSA in a recA1 host. Recombination also occurs between two intramolecular RSA sites. Unlike site-specific recombination systems encoded by other plasmids, Pre-RSA is not involved in plasmid maintenance.

Partial characterization of the genetic basis for sucrose metabolism and nisin production in Streptococcus lactis

We attempted to identify the genetic loci for sucrose-fermenting ability (Suc+), nisin-producing ability (Nip+), and nisin resistance (Nisr) in certain strains of Streptococcus lactis. To obtain genetic evidence linking the Suc+ Nip+ Nisr phenotype to a distinct plasmid, both conjugal transfer and transformation were attempted. A conjugation procedure modified to protect the recipients against the inhibitory action of nisin allowed the conjugal transfer of the Suc+ Nip+ Nisr marker from three Suc+ Nip+ Nisr donors to various recipients. The frequency of transfer ranged from 1.7 x 10(-4) to 5.6 x 10(-8) per input donor, depending on the mating pair. However, no additional plasmid DNA was apparent in these transconjugants. Transformation of S. lactis LM0230 to the Suc+ Nip+ Nisr phenotype by using the plasmid pool of S. lactis ATCC 11454 was not achieved, even though other plasmids present in the pool were successfully transferred. However, two results imply the involvement of plasmid DNA in coding for the Suc+ Nip+ Nisr phenotype. The Suc+ Nip+ Nisr marker was capable of conjugal transfer to a recipient deficient in host-mediated homologous recombination (Rec-), and the Suc+ Nip+ Nisr marker exhibited bilateral plasmid incompatibility with a number of lactose plasmids found in S. lactis. Although our results indicate that the Suc+ Nip+ Nisr phenotype is plasmid encoded, no physical evidence linking this phenotype to a distinct plasmid was obtained.

Autogenous transduction of phi 11de in Staphylococcus aureus: transfer and genetic properties

The staphylococcal plasmid phi 11de is capable of transduction in the absence of both a helper bacteriophage and detectable plaque-forming bacteriophage. The mechanism of transfer is distinct from generalized transduction in that it does not transduce chromosomal material and is selective with respect to the plasmid DNA that is transduced. The transductants containing phi 11de have the following characteristics: (i) erythromycin resistance at levels displayed by the donor, (ii) expression of and susceptibility to plasmid incompatibility, (iii) dependence upon the host recombination system during transduction, (iv) complementation of phi 11 mutants, and (v) reactivation of UV-irradiated phage.

Staphylococcus aureus chromosomal mutation specifically affecting the copy number of Inc3 plasmids

A chromosomal mutation leading to an important increase in the copy number of plasmid pT181 and its derivatives has been isolated from Staphylococcus aureus strain 8325. The amplification effect in the mutant strain SA1350 was found to be specific for plasmids of the Inc3 group, to which belongs pT181. There are some other differences in the behavior of Inc3 plasmids between SA1350 and 8325, including stable maintenance in SA1350 at high copy number of temperature-sensitive replication mutants at restrictive temperatures, and altered incompatibility properties. Derivatives of SA1350 carrying only Inc3 plasmid mutants with high copy numbers (Cop mutants) could not be obtained, suggesting a lethal runaway plasmid replication in this situation. SA1350 expressed also a temperature-sensitive phenotype. The relationship of this character to the plaC1 mutation determining the amplification of Inc3 plasmids has not yet been elucidated.

Analysis of plasmids in nosocomial strains of multiple-antibiotic-resistant Staphylococcus aureus

Nosocomial infections caused by Staphylococcus aureus strains resistant to methicillin and multiple antibiotics have reached epidemic proportions in Melbourne, Australia, over the past 5 years. Plasmid analysis of representative clinical isolates demonstrated the presence of three classes of plasmid DNA in most strains. Resistance to gentamicin, kanamycin, and tobramycin was usually mediated by an 18-megadalton plasmid but could also be encoded by a related 22-megadalton plasmid. Two distinguishable plasmids of 3 megadaltons each endowed resistance to chloramphenicol, and the third class consisted of small plasmids, each approximately 1 megadalton in size, with no attributable function. An extensive array of resistance determinants, including some which have usually been associated with a plasmid locus, were found to exist on the chromosome. Evidence that resistance to gentamicin, kanamycin, and tobramycin is chromosomally encoded in some clinical isolates suggests that this determinant may have undergone genetic translocation onto the staphylococcal chromosome.

Construction and propagation of deletion derivatives of staphylococcal tetracycline-resistance plasmid pTP-5 in Bacillus subtilis

Tetracycline-resistance (TCr) plasmid pTP-5 (4.46 kb) from Staphylococcus aureus was cleaved with HindIII into three fragments: A (2.35 kb), B (1.57 kb) and C (0.54 kb). A deletion plasmid (3.92 kb) lacking HindIII-C fragment was obtained, and was designated pNS1. This plasmid retained the TCr phenotype and the ability to replicate autonomously in Bacillus subtilis. A restriction endonuclease cleavage map of pNS1 was constructed. Attempts to construct smaller plasmids by digesting pNS1 with BAL31 nuclease led to production of a set of deletion derivatives whose molecular sizes range from 3.75 to 2.77 kb. Through analyses of these derivatives, the regions essential for autonomous replication and expression of TCr were deduced.

Structural analysis of plasmid pSN2 in Staphylococcus aureus: no involvement in enterotoxin B production

Earlier studies have suggested the involvement of a small 1.3-kilobase plasmid, pSN2, in the production of enterotoxin D by certain Staphylococcus aureus strains. On the basis of extensive biochemical studies on pSN2, including the determination of its coding properties and its primary nucleotide sequence, we conclude that this plasmid is not in act involved in enterotoxin B production in S. aureus: although the toxin genes are apparently chromosomal, it is probable that they are part of a special genetic system such as a hitchhiking transposon.