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Herbaspirillum seropedicaeis an endophytic diazotroph, which colonizes sugar cane, wheat, rice and maize. The activity of NifA, a transcriptional activator of nifgenes in H. seropedicae, is controlled by ammonium ions through a mechanism involving its N‐terminal domain. Here we show that this domain interacts specifically in vitro with the N‐truncated NifA protein, as revealed by protection against proteolysis, and this interaction caused an inhibitory effect on both the ATPase and DNA‐binding activities of the N‐truncated NifA protein. We suggest that the N‐terminal domain inhibits NifA‐dependent transcriptional activation by an inter‐domain cross‐talk between the catalytic domain of the NifA protein and its regulatory N‐terminal domain in response to fixed nitrogen.

Herbaspirillum seropedicaeis an endophytic diazotroph, which colonizes sugar cane, wheat, rice and maize. The activity of NifA, a transcriptional activator of nifgenes in H. seropedicae, is controlled by ammonium ions through a mechanism involving its N-terminal domain. Here we show that this domain interacts specifically in vitro with the N-truncated NifA protein, as revealed by protection against proteolysis, and this interaction caused an inhibitory effect on both the ATPase and DNA-binding activities of the N-truncated NifA protein. We suggest that the N-terminal domain inhibits NifA-dependent transcriptional activation by an inter-domain cross-talk between the catalytic domain of the NifA protein and its regulatory N-terminal domain in response to fixed nitrogen.

The NifA protein is responsible for transcription activation of nifgenes in the endophytic diazotroph Herbaspirillum seropedicae. When expressed in Escherichia colithis NifA protein is unable to activate the transcription of a Klebsiella pneumoniae nifH::lacZfusion. However, a form of NifA lacking the N‐terminal domain did activate transcription and its activity was not inhibited by ammonium. In this work we show that when expressed separately, the N‐terminal domain of H. seropedicaeNifA protein can restore ammonium control of the N‐truncated NifA activity in E. coli. This effect is dependent on the relative concentrations of the N‐terminal domain and the N‐truncated protein and suggests that the N‐terminal domain behaves in this respect in a manner similar to that of NifL of the γ proteobacteria.

A human apurinic/apyrimidinic endonuclease activity, called AP endonuclease I, is missing from or altered specifically in cells cultured from Xeroderma pigmentosum group-D individuals (XP-D cells) (Kuhnlein, U., Lee, B., Penhoet, E. E., and Linn, S.(1978) Nucleic Acids Res.5, 951-960). We have now observed that another nuclease activity, UV endonuclease III, is similarly not detected in XP-D cells and is inseparable from the AP endonuclease I activity. This activity preferentially cleaves the phosphodiester backbone of heavily ultraviolet-irradiated DNA at unknown lesions as well as at one of the phosphodiester bonds within a cyclobutane pyrimidine dimer. The nuclease activities have been purified from mouse cells to yield a peptide of Mr= 32,000, whose sequence indicates identity with ribosomal protein S3. The nuclease activities all cross-react with immunopurified antibody directed against authentic rat ribosomal protein S3, and, upon expression in Escherichia coliof a cloned rat cDNA for ribosomal protein S3, each of the activities was recovered and was indistinguishable from those of the mammalian UV endonuclease III. Moreover, the protein expressed in E. coliand its activities cross-react with the rat protein antibody. Ribosomal protein S3 contains a potential nuclear localization signal, and the protein isolated as a nuclease also has a glycosylation pattern consistent with a nuclear localization as determined by lectin binding. The unexpected role of a ribosomal protein in DNA damage processing and the unexplained inability to detect the nuclease activities in extracts from XP-D cells are discussed.

The NifA protein is responsible for transcription activation of nif genes in the endophytic diazotroph Herbaspirillum seropedicae. When expressed in Escherichia coli this NifA protein is unable to activate the transcription of a Klebsiella pneumoniae nifH::lacZ fusion. However, a form of NifA lacking the N-terminal domain did activate transcription and its activity was not inhibited by ammonium. In this work we show that when expressed separately, the N-terminal domain of H. seropedicae NifA protein can restore ammonium control of the N-truncated NifA activity in E. coli. This effect is dependent on the relative concentrations of the N-terminal domain and the N-truncated protein and suggests that the N-terminal domain behaves in this respect in a manner similar to that of NifL of the γ proteobacteria.