RESUMO
The 35.5-kb ICESt1 element of Streptococcus thermophilus CNRZ368 is bordered by a 27-bp repeat and integrated into the 3' end of a gene encoding a putative fructose-1,6-biphosphate aldolase. This element encodes site-specific integrase and excisionase enzymes related to those of conjugative transposons Tn5276 and Tn5252. The integrase was found to be involved in a site-specific excision of a circular form. ICESt1 also encodes putative conjugative transfer proteins related to those of the conjugative transposon Tn916. Therefore, ICESt1 could be or could be derived from an integrative conjugative element.
Assuntos
Elementos de DNA Transponíveis , Streptococcus/genética , Proteínas Virais , Proteínas de Bactérias/genética , Proteínas de Bactérias/metabolismo , Sequência de Bases , Clonagem Molecular , Conjugação Genética , DNA Nucleotidiltransferases/genética , Integrases/genética , Dados de Sequência Molecular , Fases de Leitura Aberta/genética , Recombinação GenéticaRESUMO
A novel type II restriction and modification (R-M) system, Sth368I, which confers resistance to phiST84, was found in Streptococcus thermophilus CNRZ368 but not in the very closely related strain A054. Partial sequencing of the integrative conjugative element ICESt1, carried by S. thermophilus CNRZ368 but not by A054, revealed a divergent cluster of two genes, sth368IR and sth368IM. The protein sequence encoded by sth368IR is related to the type II endonucleases R.LlaKR2I and R.Sau3AI, which recognize and cleave the sequence 5'-GATC-3'. The protein sequence encoded by sth368IM is very similar to numerous type II 5-methylcytosine methyltransferases, including M.LlaKR2I and M.Sau3AI. Cell extracts of CNRZ368 but not A054 were found to cleave at the GATC site. Furthermore, the C residue of the sequence 5'-GATC-3' was found to be methylated in CNRZ368 but not in A054. Cloning and integration of a copy of sth368IR and sth368IM in the A054 chromosome confers on this strain phenotypes similar to those of CNRZ368, i.e., phage resistance, endonuclease activity of cell extracts, and methylation of the sequence 5'-GATC-3'. Disruption of sth368IR removes resistance and restriction activity. We conclude that ICESt1 encodes an R-M system, Sth368I, which recognizes the sequence 5'-GATC-3' and is related to the Sau3AI and LlaKR2I restriction systems.
Assuntos
Enzimas de Restrição-Modificação do DNA/genética , Enzimas de Restrição-Modificação do DNA/metabolismo , Elementos de DNA Transponíveis , Streptococcus/enzimologia , Streptococcus/genética , Sequência de Bases , Clonagem Molecular , Metilação de DNA , DNA Bacteriano/análise , DNA Bacteriano/genética , Deleção de Genes , Dados de Sequência Molecular , Recombinação Genética , Mapeamento por Restrição , Análise de Sequência de DNA , Streptococcus/virologia , Fagos de Streptococcus/patogenicidade , Fagos de Streptococcus/fisiologia , Transformação BacterianaRESUMO
The SXT element (SXT) is becoming an increasingly prevalent vector for the dissemination of antibiotic resistances in Vibrio cholerae. SXT is a member of a larger family of elements, formerly defined as IncJ plasmids, that are self-transmissible by conjugation and integrate site-specifically into the host chromosome. Comparison of the DNA sequences of SXT and R391, an IncJ element from Providencia rettgeri, indicate that these elements consist of a conserved backbone that mediates the regulation, excision/integration and conjugative transfer of the elements. Both elements have insertions into this backbone that either confer the element-specific properties or are of unknown function. Interestingly, the conserved SXT and R391 backbone apparently contains hotspots for insertion of additional DNA sequences. This backbone represents a scaffold for the mobilization of genetic material between a wide range of gram-negative bacteria, allowing for rapid adaptation to changing environments.