Variability of endonucleolytic activity indicates high genetic diversity within the natural population ofSelenomonas ruminantium

Lactate dehydrogenase gene variability among predominant lactate utilizing ruminal bacteria

The inter- and intraspecies variability of lactate dehydrogenase (ldh) gene was determined among the predominant ruminal lactate utilizing bacteria. Nearly complete nucleotide sequences of ldh gene, encoding NAD-dependent lactate dehydrogenase of three Megasphaera elsdenii and six Selenomonas ruminantium strains, were obtained and compared. Phylogenetic analyses revealed a limited variability between the ldh sequences studied. The majority of differences observed were silent mutations at the 3rd position of codons. Surprisingly, the intraspecies diversity of the ldh gene among S. ruminantium isolates was higher than the interspecies level between S. ruminantium and M. elsdenii, which strongly suggests the possibility of acquisition of this gene by horizontal gene transfer.

Spreading and mutability of Selenomonas ruminantium plasmids

Two small plasmids from Selenomonas ruminantium strain 19D were cloned in Escherichia coli and completely characterized. Sequence comparison indicated that the plasmids are similar to those reported in genetically vaguely related S. ruminantium strain S20. Small 1.4-kb plasmids pSRD191 and pONE430 are only distantly related (approximately 30 % for deduced Rep protein amino acid sequence) but possess a short highly conserved region outside rep gene. Larger plasmids pSRD192 and pONE429 possess large identical DNA regions in an otherwise dissimilar background. Recombination is proposed as an important mechanism of evolution and spreading of S. ruminantium plasmids.

pSRD191, a new member of RepL replicating plasmid family from Selenomonas ruminantium

A numerous plasmid population was detected in strain 19 of Selenomonas ruminantium. The population was found to consist of six plasmids in size ranging from 1.4 to more than 20kb. The smallest 1.4kb cryptic plasmid pSRD191 was further characterized. Sequence analysis identified a single ORF encoding the 177-residue putative replication protein (Rep191) which shared significant homology with RepL family of replication protein from Firmicutes (staphylococci and bacilli). PCR analysis and Southern hybridisation showed that pSRD191 related plasmids are frequently encountered in rumen selenomonads.

GATC-specific restriction-modification systems in ruminal bacteria

The GATC-specific restriction and modification activities were analyzed in 11 major bacterial representatives of ruminal microflora. Modification phenotype was observed in 13 out of 40 ruminal strains. MboI isoschizomeric restriction endonucleases were detected in 10 bacterial strains tested; three strains lacked any detectable corresponding endonuclease activity. The only examined strain of Mitsuokella multi-acida was found to possess a different type of endonuclease activity. This is the first report on restriction activity in ruminal treponemes M. multiacida and Megasphaera elsdenii.

Restriction and modification systems of ruminal bacteria

A high frequency of type II restriction endonuclease activities was detected in Selenomonas ruminantium but not in other rumen bacteria tested. Eight different restriction endonucleases were characterized in 17 strains coming from genetically homogeneous local population. Chromosomal DNA isolated from S. ruminantium strains was found to be refractory to cleavage by various restriction enzymes, implying the presence of methylase activities additional to those required for protection against the cellular endonucleases. The presence of Dam methylation was detected in S. ruminantium strains as well as in several other species belonging to the Sporomusa subbranch of low G + C Gram-positive bacteria (Megasphaera elsdenii, Mitsuokella multiacidus).

Plasmids of Selenomonas ruminantium and development of host-vector system

A high frequency of plasmids was detected in the rumen bacterium Selenomonas ruminantium. Plasmids 0.9-20 kb in size were detected in more than 50% tested strains. Densitometric analysis indicated that plasmid DNA could represents more than 25% of total cellular DNA. Up to six plasmids were detected in strain S. ruminantium 18. Two smallest cryptic plasmids pSRD181 and pSRD182 from this strain were cloned into Escherichia coli vector pBluescriptSK+ and partially characterized. The plasmid pSRD181 is 1.4 kb and pSRD182 is 2.0 kb. While computer analysis of pSRD181 sequence data showed high homology with replication protein of Staphylococcus aureus plasmids, the pSRD182 sequence showed no significant homology in GenBank data. Strain S. ruminantium 28 was successfully transformed with pJW1 derived plasmid pJ1B1 using ampicillin resistance gene as marker. This is the first report on transformation of selenomonads with foreign DNA.

Prevalence of CTGCAG Recognizing Restriction and Modification Systemsin Ruminal Selenomonades

Analysis of restriction and modification activities in natural population of Selenomonas ruminantium have revealed the prevalence of CTGCAG (Pst I isoschizomers) recognizing restriction and/or modification systems in these bacteria. Pst I isoschizomeric restriction endonucleases were detected in 4 out of 15 strains tested. In one strain, the Pst I isoschizomeric restriction system was accompanied by another restriction and modification system recognizing GAATTC sequence (Eco RI isoschizomer). Four other strains contained CTGCAG specific methylases which lacked cognate endo-nuclease activities. Presence of identical restriction and modification systems in both of subspecies of S. ruminantium, as well as the occurrence of Pst I isoschizomers in various combinations, indicate the possibility of horizontal transfer of genes coding for these systems.