Ytql from Bacillus subtilis has both oligoribonuclease and pAp-phosphatase activity

Oligoribonuclease is the only RNase in Escherichia coli that is able to degrade RNA oligonucleotides five residues and shorter in length. Firmicutes including Bacillus subtilis do not have an Oligoribonuclease (Orn) homologous protein and it is not yet understood which proteins accomplish the equivalent function in these organisms. We had previously identified oligoribonucleases Orn from E. coli and its human homolog Sfn in a screen for proteins that are regulated by 3 '-phosphoadenosine 5 '-phosphate (pAp). Here, we identify Ytql as a potential functional analog of Orn through its interaction with pAp. Ytql degrades RNA oligonucleotides in vitro with preference for 3-mers. In addition, Ytql has pAp-phosphatase activity in vitro. In agreement with these data, Ytql is able to complement both orn and cysQ mutants in E. coli. An ytql mutant in B. subtilis shows impairment of growth in the absence of cysteine, a phenotype resembling that of a cysQ mutant in E. coli. Phylogenetic distribution of Ytql, Orn and CysQ supports bifunctionality of Ytql.