Nitrogen fixation genes (nif K,D,H) in the filamentous nonheterocystous cyanobacterium Plectonema boryanum do not rearrange

Abstract

The organisation of the structural genes for nitrogen fixation (nif K,D and H) in a nonheterocystous, filamentous cyanobacterium Plectonema boryanum has been examined in comparison with a heterocystous cyanobacterium, Anabaena torulosa. DNA from repressed (fix^-) cultures of A. torulosa showed a discontinuous nif region spread over approximately 18 kb, an arrangement typical of the vegetative cells of heterocystous cyanobacteria. The region contained a contiguousnif DH separated fromnif K. by nearly 11 kb DNA. The intervening 11 kb DNA harboured the genexis A involved in the rearrangement ofnif K,D,H to form a cluster during differentiation of heterocysts. DNA from Plectonema boryanum had a small, contiguous nif KDH cluster spanning a region of approximately 4 kb. DNA homologous to the 11 kb excison with its residentxis A was not present. Nif hybridisation patterns of restriction digests of the DNA isolated from repressed (fix^-) or induced (fix^-) cultures ofP. boryanum were completely identical. These results unequivocally demonstrate that in the nonheterocystous cyanobacterium, unlike in the heterocystous strains, no gene rearrangement, either within the nif KDII cluster or in its vicinity, accompanies the expression of nitrogenase activity.