Inhibition of lytic induction in lysogenic cyanophyces.
When the lysogenic strain SPIcts1 of the blue-green alga Plectonema boryanum carrying a temperature-sensitive mutation in the LPP2 prophage was heated at a nonpermissive temperature in the light, a lytic cycle occurred, with production of infectious viral particles. Inhibitors of transcription, translation, and photosynthetic functions interfered with this process and produced different effects when administered at different phases of the viral cycle. The presence of the inhibitors during the temperature shift did not allow a successful induction to take place; lysogens submitted to such a process produced a normal virus yield, however, when the drugs were removed and the temperature was shifted again. Incubation with the inhibitors during the early postinduction period reduced the virus yield; at later times, however, the inhibitory action rapidly declined. When cells were induced in the presence of chloramphenicol, incubated with actinomycin, and then grown in the dark, at either permissive or nonpermissive temperatures, virus multiplication was equally inhibited. These data indicate that: (i) provirus induction in lysogenic cyanophyces relies on the synthesis of early viral proteins; (ii) induction of mRNA is unstable and becomes rapidly inactivated when its translation is prevented; and (iii) inhibition of photosynthesis prevents the induction message from being expressed. It is suggested that the SPIcts1 prophage codes for a mutated repressor, which is reversibly inactivated at a nonpermissive temperature, and that the repressor must be inactivated at the same time that the message coded for by very early genes is translated, for a successful induction of the lytic cycle.