 It was previously proposed that a specific downstream sequence "PYLIS", forming a stem-loop in the mRNA, forced the incorporation of pyrrolysine instead of terminating translation in methanogenic archaea.However, the PYLIS model has lost favor in view of the lack of structural homology between PYLIS elements and the lack of UAG stops in those species.The tRNA(CUA) can be charged with lysine "in vitro" by the concerted action of the "M. barkeri" Class I and Class II Lysyl-tRNA synthetases, which do not recognize pyrrolysine.