A LIGHT-REPRESSED TRANSCRIPT FOUND IN SYNECHOCOCCUS PCC-7002 IS SIMILAR TO A CHLOROPLAST-SPECIFIC SMALL-SUBUNIT RIBOSOMAL-PROTEIN AND TO A TRANSCRIPTION MODULATOR PROTEIN ASSOCIATED WITH SIGMA-54

The gene encoding a novel light-repressed transcript (lrtA) contained within a 2.7-kbp EcoRI fragment has been cloned and sequenced from the unicellular cyanobacterium, Synechococcus PCC 7002. Northern analysis indicates that this transcript is synthesized rapidly in the dark, bu t upon 20 min of illumination, transcript levels fall below detectable limits. An open reading frame was located 378 bases from the start of the transcript which encodes a 21-kDa protein with significant homolo gy to two hitherto different proteins. The protein sequence LrtA showe d 37% sequence identity and 58% sequence similarity to the chloroplast -specific small subunit ribosomal protein, S30, and 37% sequence ident ity and 60% sequence similarity to the reported transcription modulato r protein of sigma 54 found in Klebsiella pneumonia and Azotobacter vi nelandii. Expression of the lrtA gene product is not detectable within 1 h after placing the cells in the dark, however, within 2.5 min of i llumination, [S-35]methionine incorporated into a 21-kDa protein. To a lessor extent, [S-35]methionine incorporation into a 17- and a 14-kDa protein was also seen which was followed by two other recognizable wa ves of translation at 5 and 10 min. This incorporation was not blocked by rifampicin added to dark-adapted cells prior to illumination. [S-3 5]Methionine pulsed-labeling experiments suggested that the translatio n of lrtA occurred only during the first 10 min of reillumination of d ark-adapted cells. The loss of initial [S-35]methionine labeling in th e light of the 21-kDa protein in a kanamycin-interrupted lrtA gene mut ant suggests that the lrtA codes for the 21-kDa protein.