Coexistence of endonuclease and exonuclease activities in a novel RecJ from Bacillus cereus

Biochemical characterisation of UvrD helicase and RecJ exonuclease from Neisseria gonorrhoeae

UvrD helicase and RecJ exonuclease play a critical role in DNA repair and recombination process thereby involved in the maintenance of the genomic integrity. In addition to DNA repair pathways, UvrD helicase plays an important role in phase variation and maintenance of virulence in pathogenic bacteria while RecJ is a single-stranded DNA -specific 5'-3' exonuclease activity responsible for generating a long 3'ssDNA gap for DNA resynthesis by DNA ploymerases in mismatch repair (MMR). In spite of being vital for performing these functions, there have been few reports on the mismatch repair pathway in pathogenic bacteria and particularly the interplay of mismatch repair proteins in methylation independent mismatch repair. Purified UvrD helicase from Neisseria gonorrhoeae (FA1090) (NgoUvrD) exhibits 3'-5' polarity on ssDNA and unwinds blunt end duplex DNA as well as different DNA substrates with overhangs. While NgoUvrD binds to Ni2+, Mg2+. Mn2+, Zn2+ and Ca2+, only Mg2+ and Mn2+ support the helicase activity as well as ATPase activity. Interestingly, Zn2+ inhibits both the helicase as well as ATPase activity. ssDNA binding to NgoUvrD abrogates the inhibition by Zn2+. This study, for the first time reveals a unique role of zinc in regulating UvrD helicase activity in N. gonorrhoeae. RecJ exonuclease from Neisseria gonorrhoeae is a 566 amino acid protein that contains the characteristic motifs conserved among all RecJ homologs. Site-directed mutagenesis in the conserved DHH motif abrogated enzymatic activity in D160A and H161A mutants. Interestingly, substitution of histidine 161 with alanine or serine residues enhanced RecJ exonuclease activity while the corresponding mutation in other bacterial RecJs abrogated the activity. NgoRecJ degrades double-stranded DNA with 2, 4, 6 and 8 nucleotide 5' overhang substrates unlike E. coli RecJ which degrades ssDNA with 6-nts overhang. In the present investigation we have studied the interaction between UvrD helicase and RecJ proteins participating in methylation-independent MMR pathway. Our studies highlight novel properties of NgoUvrD and NgoRecJ proteins and specific interaction between these proteins which could play in genome maintenance, pathogenesis and virulence of Neisseria gonorrhoeae.

Riemerella anatipestifer AS87_RS02955 Acts as a Virulence Factor and Displays Endonuclease Activity

Riemerella anatipestifer is an important bacterial pathogen in the global duck industry and causes heavy economic losses. In our previous study, we demonstrated that R. anatipestifer type IX secretion system components GldK and GldM, and the secretion protein metallophosphoesterase, acted as virulence factors. In this study, R. anatipestifer AS87_RS02955 was investigated for virulence and enzymatic activity properties. We constructed AS87_RS02955 mutation and complementation strains to assess bacterial virulence. In vivo bacterial loads showed a significantly reduced bacterial loads in the blood of ducks infected with mutant strain Yb2Δ02955, which was recovered in the blood of ducks infected with the complementation strain cYb2Δ02955, demonstrating that AS87_RS02955 was associated with virulence. Further studies showed AS87_RS02955 was a novel nonspecific endonuclease with no functionally conserved domain, but enzymatic activity toward DNA and RNA was indicated. DNase activity was activated by Zn²⁺, Cu²⁺, Mg²⁺, Ca²⁺, and Mn²⁺ ions but inhibited by ethylenediaminetetraacetic acid. RNase activity was independent of metal cations, but stimulated by Mg²⁺, Ca²⁺, and Mn²⁺. RAS87_RS02955 enzymatic activity was active across a broad pH and temperature range. Moreover, we identified four sites in rAS87_RS02955, F39, F92, I134, and F145, which were critical for enzymatic activity. In summary, we showed that R. anatipestifer AS87_RS02955 encoded a novel endonuclease with important roles in bacterial virulence. IMPORTANCE R. anatipestifer AS87_RS02955 was identified as a novel T9SS effector and displayed a nonspecific endonuclease activity in this study. The protein did not contain a conserved His-Asn-His motif structure, which is similar to the endonuclease from Prevotella sp. Its mutant strain Yb2Δ02955 demonstrated significantly attenuated virulence, suggesting AS87_RS02955 is an important virulence factor. Moreover, AS87_RS02955 displayed nonspecific endonuclease activity to cleave λ DNA and MS2 RNA, while four protein sites were critical for endonuclease activity. In conclusion, R. anatipestifer AS87_RS02955 plays important roles in bacterial virulence.