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It has long been known that while strong E. coli promoters work well in cyanobacteria, the reverse is often not the case. The question remains open as to what cyanobacterial RNA polymerases look for to initiate transcription. Wipa Chungjatupornchai tells us of work she did with Thanaset Senawong and Sakol Panyim at Mahidol University trying to define what functions as a promoter in Synechococcus PCC 7942. They placed random genomic fragments from Synechococcus upstream from a promoterless GUS (ß-glucuronidase) gene and sent the library into Synechococcus on a plasmid capable of independent replication. Of 640 transformants, 15 expressed GUS strongly and 25 had low expression. Only 4 of these 40 clones also could express GUS in E. coli.
Two DNA fragments were selected for further study. One that promoted GUS expression in both Synechococcus and E. coli carries a reasonable -35 and -10 sequence immediately upstream from what serves as the transcriptional initiation site in both organisms.
The second fragment promoted strong GUS expression but only in Synechococcus. Transcription was initiated in the cyanobacterium immediately downstream from what appears to be a tRNApro gene. There is nothing upstream resembling a sigma-70 recognition sequence, but, curiously, the upstream sequence bears a strong resemblance to the major promoter elements in eukaryotic tRNA genes.
The work will be published soon in Current Microbiology.