Light-mediated transcriptional regulation and genome mapping in the cyanobacterium Synechococcus PCC 7002

The unicellular cyanobacterium Synechococcus PCC 7002 which possesses oxygenic photosynthesis similar to that of higher plants was chosen for genome mapping studies. A contiguous EcoRI restriction map from overlapping cosmid clones of the Synechococcus genome was constructed from a cosmid library, consisting of 566 clones. Individual cosmid clones were grouped into contigs by hybridization to macrorestriction fragment probes or specific gene probes. The patterns of the restriction digests of grouped cosmid clones (ensembles) were compared on field inversion gel electrophoresis and the contig map was build. Contigs were positioned by hybridization within large PFGE fragments. The exact position of a contig on the genomic map was determined either by locating specific genome markers or by identifying cosmids containing unique NotI and or SalI restriction sites. Chromosome walking was used to connect adjacent contigs. To date, 95% of the Synechococcus genome has been covered with cosmid clones. The map has been very useful in locating and ordering genes of interest and it will be used as the basis for sequencing the entire genome.; lrtA is a gene encoding a novel light-repressed transcript that has been cloned from the unicellular cyanobacterium Synechococcus PCC 7002. The light mediated regulation of lrtA has been investigated in this study. lrtA transcript levels were monitored with Ribonuclease Protection Assays. The transcript for lrtA was not detected in continuously illuminated cells but transcript levels increased when the cells were placed in the dark. Transcript synthesis continued in the dark for at least 3 hr and the transcript levels remained elevated for at least 7 hr. lrtA transcript levels remained constant for over 30 min after rifampicin was added to 3 hr dark adapted cells, however, in the light, the transcript's half-life was 4 min. RNase activity found in lysates from illuminated or dark adapted cells showed little differences suggesting that decreased RNase activity was not the reason for the observed lrtA message stability in the dark. The addition of chloramphenicol to illuminated cells allowed the detection of the transcript in the light indicating that lrtA transcript repression depends upon new protein synthesis. The wavelength dependence of lrtA repression in the light was determined. A bimodal action spectrum was obtained exhibiting repression at the blue and red region of the visible electromagnetic spectrum. The bimodal nature of the lrtA transcription dependence spectrum suggests the involvement of a chlorophyll like pigment protein receptor that senses and transmits the light signal independently of the photosynthetic energy production.