Genetics Molecular Biology>Viral Replication Life Cycles Practice Test

Researchers infected a culture of Escherichia coli with a nonenveloped double-stranded DNA bacteriophage. Adsorption to specific outer membrane porins was followed by injection of the viral genome. Within minutes, host DNA replication and transcription were halted, and nucleases degraded bacterial chromosomes. Early viral genes encoded nucleases and a DNA-dependent DNA polymerase; late genes encoded capsid proteins and tail fibers. Electron micrographs showed assembly of progeny particles in the cytoplasm. By 25 minutes postinfection, the culture's optical density abruptly decreased, and plaque assays revealed a burst size of about 200 particles per cell. No evidence of integration into the host chromosome was observed, and infected cells did not divide before virus release. In separate experiments, inhibition of peptidoglycan synthesis did not prevent virus release. Instead, enzymatic activity consistent with holins and endolysins was detected at the time of viral release, and culture supernatants contained abundant infectious particles shortly after infection. Host viability rapidly declined.

Investigators examined a temperate bacteriophage that infects Salmonella enterica. After infection at low multiplicity, individual bacterial colonies continued to grow and divide with no immediate loss of viability. Southern blot analysis detected viral DNA integrated at a specific att site adjacent to a host metabolic operon. Viral transcription was limited to a single repressor that prevented expression of structural proteins. Infected cells became resistant to superinfection by the same phage. When cultures were exposed briefly to ultraviolet light or treated with a DNA-damaging agent, the viral genome excised, late genes were expressed, and within an hour many cells lysed and released infectious particles. Prior to induction, no free virions were detected in the medium, and the phage genome was replicated passively as the host chromosome duplicated during cell division. The phage conferred a selectable marker to host cells, and this trait was stably inherited until induction occurred.