Toxic component of a type II toxin-antitoxin (TA) system (PubMed:20980243, PubMed:24112600). A stable (half-life 27.6 minutes) endoribonuclease that in the absence of cognate antitoxin RnlB causes generalized RNA degradation. Degrades late enterobacteria phage T4 mRNAs, protecting the host against T4 reproduction. Activity is inhibited by cognate antitoxin RnlB and by enterobacteria phage T4 protein Dmd (PubMed:20980243, PubMed:22403819). Targets cyaA mRNA (PubMed:19019153). Forms homodimer in solution (PubMed:24112600). Forms a complex with cognate antitoxin RnlB (PubMed:20980243) and with enterobacteria phage T4 antitoxin Dmd (PubMed:22403819). May associate with ribosomes, it sediments in a P100 fraction (pellet of a 100,000 x g centrifugation). Repressed by IscR. rnlA-rnlB forms an operon, the downstream rnlB also has its own promoter. Composed of 3 domains, the NTD (N-terminal domain, residues 1-90), NRD (N-repeated domain, residues 91-197) and DBD (Dmd-binding domain, residues 198-357). The DBD is responsible for dimerization, growth inhibition upon overexpression and recognition of T4 antitoxin Dmd (via residues 327-357) and antitoxin RnlB and subcellular location (PubMed:24112600). Not essential, slightly smaller colonies on minimal agar plates at 30 degrees Celsius. Strongly reduces RNase LS activity, restores growth of an enterobacteria phage T4 dmd mutant. Altered mRNA half-life for some cellular transcripts, including an increased half-life for an internal fragment of 23S rRNA. Allele rnlA2 is due to a premature stop codon at residue 33 (PubMed:15677746). Sensitive to high concentrations of NaCl, sensitivity is eliminated in a crp or cyaA deletion mutant. Increased levels of Crp and thus increased levels of cAMP.