Among 14 temperature-sensitive, growth-defective mutants of mycobacteriophage L1 showing a lysis-defective phenotype at 42 degrees C, six are, in addition, defective in phage DNA synthesis at 42 degrees C. In the present study, we show that one of the latter six mutants, L1G27ts901, is also defective in the synthesis of both an L1-specific exonuclease (a representative delayed early protein), and of RNA in both the delayed early and late periods but not in the immediate early period. The results of a temperature-shift experiment suggest that the synthesis of L1 exonuclease is regulated by G27 at the level of transcription. Furthermore, the temperature-sensitive defect in delayed early and late RNA synthesis could be largely overcome when the L1G27ts901-infected culture was shifted from 32 to 42 degrees C at 10 min but not at zero time post-infection. These results suggest that the primary effect of the G27ts901 mutation is to make the phage defective in transcription of delayed early genes at 42 degrees C, and the defect in late RNA synthesis by this mutant is a secondary effect which is caused by its inability to express regulatory gene products. We conclude that G27 is involved in the positive regulation of expression of the delayed early genes of L1 at the transcriptional level.


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