Promoter recognition by the RNA polymerase from vegetative cells of the cyanobacterium Anabaena 7120.

The transcription start points (tsp) of seven genes of Anabaena 7120 were previously identified by S1 nuclease protection and primer extension experiments using RNA extracted from cells. In the present work, these tsp were confirmed, with one exception, by in vitro transcription using purified RNA polymerases of Anabaena 7120 and Escherichia coli, and crude extracts of Anabaena 7120 active in transcription. In all cases, the template for transcription consisted of closed circular plasmid DNA in which the putative promoter-containing fragment was cloned in front of a strong terminator, which resulted in defined 'pseudo-runoff' transcripts whose sizes correspond (with one exception) to those expected on the basis of the tsp determined for in vivo RNA. These results, together with others obtained with templates containing bacteriophage T4 or cyanophage N1 promoters, led to the conclusion that the principal Anabaena 7120 RNA polymerase prefers promoters whose sequence and spacing approximate that of the E. coli consensus promoter, and that the Anabaena 7120 genes expressed in vegetative cells, characterized to date, have relatively weak promoters.