Purification and characterization of Escherichia coli RNase I. Comparisons with RNase M.

The endoribonuclease, RNase I, was purified from the periplasm of Escherichia coli. Based on PAGE, it has molecular mass of approximately 27 kDa with a migration rate indistinguishable from that of the recently reported RNase M from E. coli. The amino acid sequence of the two enzymes must be very similar based on two-dimensional mapping of their tryptic peptides and suggests either a post-transcriptional modification to yield different proteins from the same gene or evolution of two genes by gene duplication. However, while RNase I could degrade each of the four ribonucleotide homopolymers, only poly(U) or poly(C) were good substrates for RNase M with possibly some hydrolysis of poly(A). The reaction rate for poly(C) hydrolysis with RNase M was about ten times faster than for poly(U), while for RNase I the rates were about equal. Besides differences in specificity, RNase M was only located in the spheroplasts while RNase I found in the periplasm of growing cells. In terms of function, RNase I is known to cause degradation of rRNA during periods of stress or non-growth, whereas it has been proposed that RNase M is the endonuclease for mRNA degradation in growing cells.