The nuiA gene from Anabaena sp. encoding an inhibitor of the NucA sugar-non-specific nuclease.

Many filamentous, heterocyst-forming cyanobacteria express a sugar-non-specific nuclease of about 29 kDa that can be detected in DNA-containing SDS-PAGE gels. The nucA gene encoding this nuclease has previously been cloned from Anabaena sp. PCC 7120, sequenced and expressed in Escherichia coli. The NucA protein bears a putative signal peptide close to its N-terminal end and, in Anabaena cultures, is present in both the cells and the extracellular medium. Cell-free extracts of different cyanobacteria producing NucA-like nucleases exhibited an inhibitory activity on NucA. In Anabaena sp. PCC 7120, this inhibition was exerted by protein(s) or protein-containing molecule(s) that were heat resistant. Immediately downstream from the nucA gene, in the complementary strand, we have identified an open reading frame composed of 135 codons, that we have named nuiA, whose expression in E. coli conferred heat-resistant NucA-inhibitory activity to cell-free extracts. The NuiA protein was purified to homogeneity, and purified NuiA inhibited the nuclease activity of NucA. Sequences hybridizing with the nuiA gene have been found in all the tested cyanobacterial strains that express a NucA-like nuclease. Whereas the NucA protein is homologous to endonuclease G from vertebrates and to nucleases from Serratia marcescens and yeast, no protein homologous to NuiA was found in the available databases. Therefore, nuiA represents a novel gene encoding a nuclease inhibitor.