Cryptic plasmids inClostridium botulinumandC. botulinum-like organisms

Evolution of Chromosomal Clostridium botulinum Type E Neurotoxin Gene Clusters: Evidence Provided by Their Rare Plasmid-Borne Counterparts

Analysis of more than 150 Clostridium botulinum Group II type E genomes identified a small fraction (6%) where neurotoxin-encoding genes were located on plasmids. Seven closely related (134–144 kb) neurotoxigenic plasmids of subtypes E1, E3, and E10 were characterized; all carried genes associated with plasmid mobility via conjugation. Each plasmid contained the same 24-kb neurotoxin cluster cassette (six neurotoxin cluster and six flanking genes) that had split a helicase gene, rather than the more common chromosomal rarA. The neurotoxin cluster cassettes had evolved as separate genetic units which had either exited their chromosomal rarA locus in a series of parallel events, inserting into the plasmid-borne helicase gene, or vice versa. A single intact version of the helicase gene was discovered on a nonneurotoxigenic form of this plasmid. The observed low frequency for the plasmid location may reflect one or more of the following: 1) Less efficient recombination mechanism for the helicase gene target, 2) lack of suitable target plasmids, and 3) loss of neurotoxigenic plasmids. Type E1 and E10 plasmids possessed a Clustered Regularly Interspaced Short Palindromic Repeats locus with spacers that recognized C. botulinum Group II plasmids, but not C. botulinum Group I plasmids, demonstrating their long-term separation. Clostridium botulinum Group II type E strains also carry nonneurotoxigenic plasmids closely related to C. botulinum Group II types B and F plasmids. Here, the absence of neurotoxin cassettes may be because recombination requires both a specific mechanism and specific target sequence, which are rarely found together.

Screening for plasmids in the genus Clostridium

A plasmid screening was performed on 150 strains out of 75 clostridial species using a modification of the alkaline-lysis procedure. In 26 strains representing 21 species one or more plasmid bands were detected ranging in size from 3 to more than 100 kilobase pairs. Clostridium aceticum proved to contain a single small plasmid (pCA1) of 5.4 kbp as revealed by restriction analysis and electron microscopy. A physical map of pCA1 has been constructed. Spontaneous mutants of C. aceticum defective in autotrophic growth have been isolated. No direct correlation between plasmid content and autotrophy could be found.

Extrachromosomal systems and gene transmission in anaerobic bacteria

Obligately anaerobic bacteria are important in terms of their role as medical pathogens as well as their degradative capacities in a variety of natural ecosystems. Two major anaerobic genera, Bacteroides and Clostridium, are examined in this review. Plasmid elements in both genera are reviewed within the context of conjugal transfer and drug resistance. Genetic systems that facilitate the study of these anaerobic bacteria have emerged during the past several years. In large part, these developments have been linked to work centered on extrachromosomal genetic systems in these organisms. Conjugal transfer of antibiotic resistance has been a central focus in this regard. Transposable genetic elements in the Bacteroides are discussed and the evolution and spread of resistance to lincosamide antibiotics are considered at the molecular level. Recombinant DNA systems that employ shuttle vectors which are mobilized by conjugative plasmids have been developed for use in Bacteroides and Clostridium. The application of transmission and recombinant DNA genetic systems to study these anaerobes is under way and is likely to lead to an increased understanding of this important group of procaryotes.

Characterization of a bacteriocinogenic plasmid from Clostridium perfringens and molecular genetic analysis of the bacteriocin-encoding gene

The bacteriocinogenic plasmid pIP404 from Clostridium perfringens was isolated and cloned in Escherichia coli, and its physical map was deduced. Expression of the bcn gene, encoding bacteriocin BCN5, is inducible by UV irradiation of C. perfringens and thus resembles the SOS-regulated bacteriocin genes of enteric bacteria. The location of bcn on pIP404 was established by a dot-blot procedure, using specific hybridization probes to analyze mRNA samples from induced and uninduced cultures. From the nucleotide sequence of its gene, the molecular weight of BCN5 was deduced to be 96,591, and a protein of this size was secreted by bacteriocin-producing cultures of C. perfringens. The primary structure of the protein suggests that it may function as an ionophore, since a hydrophobic domain, resembling those of the ionophoric colicins, is present at the COOH terminus. No bacteriocin activity could be detected in E. coli harboring plasmids bearing the bcn gene, even when the transcriptional and translational signals were replaced by those of lacZ. A possible explanation may be found in the unusual codon usage of the adenine-thymine-rich bcn gene, as this shows a preference for codons with a high adenine-plus-thymine content, especially in the wobble position. Many of the frequently used codons correspond to those recognized by minor tRNA species in E. coli. Consequently, bcn expression might be limited by tRNA availability in this bacterium.

Plasmid detection and isolation in strains of Clostridium acetobutylicum and related species

Twenty-one strains of Clostridium acetobutylicum, C. butylicum and Clostridium saccharoperbutylacetonicum were examined. Seven of them contained extrachromosomal DNA molecules, with a size ranging from 2.6 to more than 50 megadaltons. Strain M1 carries a small plasmid of 2.6 megadaltons, AB10 at least one plasmid of 2.6 megadaltons, AB12 one plasmid of 5.2 megadaltons, AB14 and AB16 a plasmid of about 7 megadaltons and a large one of more than 50 megadaltons, AB17 carries at least one plasmid of 6.7 megadaltons and AB18 two plasmids (4-6 megadaltons and 10-12 megadaltons). All of them are cryptic as at present no function can be correlated with their presence in a bacterial strain.