RESUMO
The Pol II-type mechanism is conserved from yeast to human. After initiation of transcription, Pol II can be paused within the early transcribed region of a gene. Then Pol II overcomes the initial nucleosomal barrier, and efficiently proceeds through chromatin. At low- to moderate-level transcription progression of Pol II is characterized by displacement/exchange of only H2A/H2B dimer(s) and hexasome survival, likely mediated through formation of small intranucleosomal DNA loops. This mechanism helps to preserve the "histone" code during transcription. As the transcription rate is increased, the distance between transcribing Pol II complexes becomes shorter, and trailing Pol II complexes may encounter the hexasome formed after previous transcription round, before the H2A/H2B dimer re-binds to the hexasome. In this case an unstable intermediate with a smaller number of DNA-histone contacts is formed, resulting in eviction of the histone hexamer from DNA in vitro; therefore here all core histones are evicted/exchanged in vivo. Various protein factors and histone chaperones are involved in chromatin transcription by Pol II in vivo.
Assuntos
Cromatina/genética , RNA Polimerase II/genética , Elongação da Transcrição Genética , Transcrição Gênica , Núcleo Celular/genética , Proteínas de Ligação a DNA/genética , Proteínas de Grupo de Alta Mobilidade/genética , Histonas/genética , Histonas/metabolismo , Humanos , Nucleossomos/genética , Fatores de Elongação da Transcrição/genéticaRESUMO
The properties of the isolated Pseudomonas aeruginosa bacteriophage phiPMG1 include the lytic infection cycle, and the formation of a broad halo (semi-transparent zone) around the plaques. We consider phiPMG1 as a potential member of therapeutic cocktails of live phages, and as a source of peptidoglycan and lipopolysaccharide degrading enzymes. Partial sequencing of phiPMG1 genome has revealed high similarity with known temperate P. aeruginosa phage D3. An open reading frame encoding lytic transglycosilase was identified in the genome. This enzyme PMG MUR was obtained in recombinant form, and its activity and substrate specificity has been studied.
Assuntos
Bacteriófagos/enzimologia , N-Acetil-Muramil-L-Alanina Amidase/genética , Pseudomonas aeruginosa/virologia , Sequência de Aminoácidos , Bacteriófagos/ultraestrutura , Estabilidade Enzimática , Genoma , Humanos , Dados de Sequência Molecular , N-Acetil-Muramil-L-Alanina Amidase/química , N-Acetil-Muramil-L-Alanina Amidase/metabolismo , Pseudomonas aeruginosa/genética , Análise de Sequência de DNARESUMO
Lytic transglycosylases are abundant peptidoglycan lysing enzymes that degrade the heteropolymers of bacterial cell walls in metabolic processes or in the course of a bacteriophage infection. The conventional catalytic mechanism of transglycosylases involves only the Glu or Asp residue. Endolysin gp144 of Pseudomonas aeruginosa bacteriophage phiKZ belongs to the family of Gram-negative transglycosylases with a modular composition and C-terminal location of the catalytic domain. Glu115 of gp144 performs the predicted role of a catalytic residue. However, replacement of this residue does not completely eliminate the activity of the mutant protein. Site-directed mutagenesis has revealed the participation of Tyr197 in the catalytic mechanism, as well as the presence of a second active site involving Glu178 and Tyr147. The existence of the dual active site was supported by computer modeling and monitoring of the molecular dynamics of the changes in the conformation and surface charge distribution as a consequence of point mutations.