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-containi ng SDS-PAGE gels. The nucA gene encoding this nuclease has previously been cloned from Anabaena sp. PCC 7120, sequenced and expressed in Esc herichia cell. The NucA protein bears a putative signal peptide close to its N-terminal end and, in Anabaena cultures, is present in both th e cells and the extracellular medium. Cell-free extracts of different cyanobacteria producing NucA-like nucleases exhibited an inhibitory ac tivity on NucA. Ln Anabaena sp. PCC 7120, this inhibition was exerted by protein(s) or protein-containing molecule(s) that were heat resista nt. Immediately downstream from the nucA gene, in the complementary st rand, we have identified an open reading frame composed of 135 codons, that we have named nuiA, whose expression in E. coli conferred heat-r esistant NucA-inhibitory activity to cell-free extracts. The NuiA prot ein was purified to homogeneity, and purified NuiA inhibited the nucle ase activity of NucA. Sequences hybridizing with the nuiA gene have be en 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. (C) 1997 Academic Press Limited.