Sequences downstream from the transcriptional start site are essential for microaerobic, but not symbiotic, expression of the Rhizobium meliloti nifHDK promoter

Summary Deletion analysis studies have been carried out on the nifHDK promoter (p1) of R. meliloti in the expression of this promoter under both free-living microaerobic and symbiotic conditions. Deletion of a region down-stream (+17 to +61) from the promoter element resulted in low levels of expression under free-living microaerobic conditions However, wild-type levels of expression were obtained during symbiosis with Alfalfa plants. The sequences in this region were designated the‘downstream sequences'. The pattern of expression observed when the downstream sequences were deleted was similar to that observed when a previously identified upstream activator sequence (UAS) was deleted. Only when both the downstream sequences and the UAS were deleted, did activity form the P1 promoter become significantly decreased during symbiosis. Expression studies of the P1 promoter in a nifA mutant background indicate that nifA is required for symbiotic expression of P1 which is enhanced by the presence of the downstream sequences.