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Background  Orthologous proteins of the Crp/Fnr family have been previously implicated in controlling expression and/or activity of the NifA transcriptional activator in some diazotrophs. This study aimed to address the role of three Fnr-like proteins from  H. seropedicae  SmR1 in controlling NifA activity and consequent NifA-mediated transcription activation.    Results  The activity of NifA-dependent transcriptional fusions ( nifA::lacZ  and  nifB::lacZ ) was analysed in a series of  H. seropedicae fnr  deletion mutant backgrounds. We found that combined deletions in both the  fnr1  and  fnr3  genes lead to higher expression of both the  nifA  and  nifB  genes and also an increased level of  nifH  transcripts. Expression profiles of  nifB  under different oxygen concentrations, together with oxygen consumption measurements suggest that the triple  fnr  mutant has higher respiratory activity when compared to the wild type, which we believe to be responsible for greater stability of the oxygen sensitive NifA protein. This conclusion was further substantiated by measuring the levels of NifA protein and its activity in  fnr  deletion strains in comparison with the wild-type.    Conclusions  Fnr proteins are indirectly involved in controlling the activity of NifA in  H. seropedicae,  probably as a consequence of their influence on respiratory activity in relation to oxygen availability. Additionally we can suggest that there is some redundancy in the physiological function of the three Fnr paralogs in this organism, since altered respiration and effects on NifA activity are only observed in deletion strains lacking both  fnr1  and  fnr3 .    Electronic supplementary material  The online version of this article (doi:10.1186/s12866-015-0432-6) contains supplementary material, which is available to authorized users.

Enhanced oxygen consumption in Herbaspirillum seropedicae fnr mutants leads to increased NifA mediated transcriptional activation