Isolation and complementation of temperature-sensitive replication mutants of Staphylococcus aureus plasmid pC194

Molecular structure of the replication origin of a Bacillus subtilis (natto) plasmid, pUH1

The structure of a 2.0-kb BstEII DNA sequence necessary and sufficient for the replication of a 5.7-kb Natto plasmid, pUH1, which is responsible for gamma-polyglutamate production by Bacillus subtilis (natto), has been characterized by using a trimethoprim resistance gene derived from B. subtilis chromosomal DNA as a selective marker. The 2.0-kb DNA sequence contains an open reading frame, rep, stretching for 999 bp; a promoter region for rep expression; and a possible replication origin for the plasmid upstream of the promotor. The predicted Rep protein has highly homologous amino acid sequences with rep14 of pFTB14 in B. amyloliquefaciens, RepB of pUB110, and protein A, which is necessary for pC194 replication in staphylococci throughout the protein molecule, but is not homologous with RepC of staphylococcal plasmid pT181.

Plasmid structural instability associated with pC194 replication functions

The hybrid plasmid pJS37 is composed of the streptococcal plasmid pLS1, which confers tetracycline resistance, and the staphylococcal plasmid pC194, which confers chloramphenicol resistance. When gram-positive bacteria containing pJS37 were grown in the presence of chloramphenicol, four different deleted derivatives accumulated. The deletions in the plasmid enhanced resistance to chloramphenicol by placing the cat gene of pC194 near promoters of pLS1. All four deletions shared a common endpoint that corresponded to the putative target site for DNA strand nicking by the pC194 replication protein, RepH. At the other, variable endpoint, the DNA sequence was similar to the putative RepH target sequence. Alteration of the RepH protein, by in vitro modification of the gene encoding it, eliminated this class of deletions. By extending a previously proposed model for the generation of a different but related class of deletions (B. Michel and S.D. Ehrlich, EMBO J. 5:3691-3696, 1986), a comprehensive model that could generate both classes of deletions is suggested. It proposes that a nicking-closing activity of the plasmid replication protein at its normal target site and, aberrantly, at sites with similar sequence can generate deletions either proximal or distal to the aberrant site during rolling-circle replication of the plasmid.

Initiation of plasmid pC194 replication and its control in Bacillus subtilis

By deletion analysis we have defined a 1.1 kb segment required for driving autonomous replication of the plasmid pC194. The minimal replicon specifies a positive, RepH, and a negative, Inc8A, trans-acting product and their target sites. The RepH product has a Mr of 34.1 kDa, could be overproduced, and binds specifically to the pC194 origin region. By trans complementation studies we have shown that pC194 replication is indirectly controlled at the level of RepH synthesis by a negative product, IncA, that is transcribed within the repH transcription unit in the opposite direction ("antisense RNA"). The antisense RNA regulates the RepH synthesis by a mechanism that seems to involve RNA/RNA interaction in a manner that interferes with translation. In addition, an autoregulatory control might be operative.

Molecular structure of the replication origin of a Bacillus amyloliquefaciens plasmid pFTB14

The structure of a 1.5-kb DNA sequence that is necessary and sufficient for the replication of an 8.2-kb cryptic plasmid, pFTB14, isolated from a strain of Bacillus amyloliquefaciens has been characterized. The 1.5-kb DNA sequence contains an open reading frame, rep, stretching for 1017 bp, a promoter region for rep expression, and a possible replication origin for the plasmid upstream of the promoter. The rep product is trans-active and essential for plasmid replication. The predicted rep protein is a basic protein, as are the RepC protein of pT181, RepB of pUB110 and protein A of pC194 (all these found in staphylococci) and the pi protein of the R6K plasmid of Escherichia coli. The predicted rep protein has highly homologous amino acid sequences with protein A of pC194 and RepB of pUB110 throughout the protein molecule, but not with RepC of pT181, pi of R6K or protein RepH encoded by and initiating the replication of pC194.

Thermostability and superhelicity of plasmid DNA in Bacillus stearothermophilus

The thermostability of the staphylococcal plasmids pC194 and pUB110 and their antibiotic-resistance determinants was examined upon transfer to Bacillus stearothermophilus CU21. Plasmid pGS13, a pUB110 derivative carrying the chloramphenicol acetyltransferase (CAT) gene of pC194, could be maintained up to the maximum growth temperature (68 degrees C) by selection for chloramphenicol resistance. In the absence of selective pressure, pGS13 was lost at temperatures above 60 degrees C. Segregational instability of pGS13 was accompanied by a progressive loss of negative superhelicity at elevated temperatures. Thermostable mutants of pGS13 were isolated by screening for expression of the antibiotic-resistance determinants after growth under non-selective conditions. These mutants were found to contain an insertion of a 1.7 kb DNA sequence derived from the cryptic B. stearothermophilus plasmid pBS02. Increased thermostability correlated with preservation of plasmid superhelicity at elevated temperatures.

The Inc3B determinant of plasmid pT181. A mutational analysis

A region encompassing the origin of replication of staphylococcal plasmid pT181 has previously been shown to express an incompatibility effect denoted Inc3B, when cloned into another replicon (Novick et al. 1984). In an attempt to understand the mechanism of this incompatibility effect, and its relationship with the function of the replication origin, mutants deficient in this property were isolated and characterized. The results obtained suggest that the Inc3B effect is due to the competition for replication between the replication origin cloned in a hybrid and the origin of an autonomous plasmid. The Inc3B-deficient mutants isolated expressed different degrees of residual incompatibility. The inc3B mutations which did not express any incompatibility were found also to inactivate the function of the replication origin. All the other mutants which expressed residual Inc3B had a functional origin but presented a significantly reduced ability to use this origin when coexisting with a plasmid using a wild-type pT181 origin. It is suggested that these inc3B mutations represent a new type of origin mutation which affects the ability of the origin to compete with other origins using the same replication system, though the function per se of the origin is not significantly impaired.

Cloning of DNA segments of phage 2C, which allows autonomous plasmid replication in Bacillus subtilis

The chromosome of Bacillus subtilis phage 2C is a 100-MDa double-stranded DNA molecule, containing hydroxymethyluracil in place of thymine and carrying redundant ends each encompassing 10% of the genome. 2C DNA was cleaved with EcoRI and HindIII, and cloned in the shuttle plasmids pSC 540 and pCP 115, both containing segments originating from B. subtilis and Escherichia coli plasmids. These chimaerical plasmids, carrying the chloramphenicol resistance gene, were unable to replicate in B. subtilis; this ability was restored, however, after the insertion of viral DNA segments. Physical maps of the recombinant plasmids were made; a large deletion of the E. coli-derived segment of pSC 540 was observed (which paralleled a loss of replication in this host), whereas addition of 2C DNA segments in pCP 115 was not accompanied by deletion (replication in E. coli was conserved in this case). Cloned viral segments mapped mostly, but not exclusively, within the redundant ends of 2C DNA. It is suggested that the thirteen recombinant clones carried the replication origin region of phage 2C DNA, and that these sequences originated within or close to the redundant extremities of the viral chromosome.