The MSMEG_4626 gene was cloned from Mycobacterium smegmatis MC2 155. It codes for a protein of 1,037 amino acids, identified as ribonuclease E by matching to the protein family HMM TIGR00757. The protein was expressed and purified. Although its calculated molecular weight is 112.7 kDa, it has an aberrant mobility in SDS-polyacrylamide gels, like other ribonuclease E enzymes (it migrates as a 180 kDa protein). The central part of the protein displays high similarity to the catalytic domains of other RNase E enzymes. Mass spectrometric analysis revealed the presence of the chaperonin GroEL, ribosomal proteins, a negative regulator of genetic competence and GTP pyrophosphokinase in the affinity-purified preparation. It is a very unstable protein; despite the use of protease inhibitors in addition to the full-length RNase E its proteolytic fragments were detected.