Differential Expression of Bacteriophage Genomes in Vegetative and Sporulating Cells of Bacillus subtilis

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Two antigenically distinct bacteriophages, β3 and β22, have been isolated and characterized with Bacillus subtilis strain W23 as a host. They differ in plaque morphology, single-step growth characteristics, host range, and thermal stability. The deoxyribonucleic acids isolated from β3 and β22 differ in base composition, density in CsCl and Cs2SO4, sedimentation coefficient, molecular weight, and thermal denaturation temperature. These phages have been used to analyze the ability of B. subtilis to sporulate despite infection by virulent phages. When development of phages β3 and β22 in sporulating cultures was compared with that in log cultures, an increase in the latent periods of infection and a decrease in the burst sizes for the two phages were observed. Sporulating cultures infected with β3 yielded the usual percentage (85%) of mature spores; 80% of these contained phage determinants and 20% were uninfected. However, cultures infected with β22 lysed. Of the small fraction (0.01%) which sporulated, 83% were uninfected and 17% were infected. Phage β3-infected and uninfected spores were examined to distinguish any chemical or physical differences. Preparations of both types of spore contained 81.4 μg of dipicolinic acid per mg (dry weight), and examination by phase-contrast microscopy gave no evidence of any difference in outward appearance. A 20% decrease in infected spore count was observed upon heating at 80 C for 10 min. Differences in the infection processes of the two phages prompted an analysis of the transcription process after infection. Deoxyribonucleic acid-ribonucleic acid hybrid analysis of relative amounts of phage-specific and host-specific messenger ribonucleic acid (mRNA) present in infected cells suggested that β3 was unable to repress the synthesis of host mRNA and that β3-specific mRNA synthesis was repressed in sporulation-phase cultures. Phage β22, in contrast, was able to repress host-specific mRNA synthesis in both log-infected and sporulation-infected cells. The results suggest that the differential expression of the phage genomes is due to the relative ability of the phages to repress the host genome.

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