text: Yo, check it out, I got some knowledge to drop
  'Bout them exoribonucleases, don't you stop
  RNase II and PNPase, they play a crucial role
  Breakin' down mRNA fragments, keepin' control
  
  We tested small RNA substrates, let me explain
  With stem-loops and extensions, nothin' plain
  These enzymes, they got exonucleolytic might
  But certain structures can impede their flight
  
  Five G-C base pairs, they don't put up a fight
  Neither enzyme gets blocked, they stay alright
  But stable stem-loops, 7, 9, or even 11 bp
  They block those enzymes, put 'em to sleep
  
  When enzyme activity is limited, listen close
  Both enzymes stall and they start to doze
  They dissociate from the substrate, step aside
  Six to nine residues, on average they slide
  
  From the base of a stable stem-loop structure
  That's the mechanism, no need for rupture
  So now you know why RNase II and PNPase
  Behave like they're redundant, it's no maze