Model presenting the effect of stress conditions on end-maturation of pre-tRNATyr and pre-tRNAUAUIle. (A) Under standard growth conditions, tRNATyr is first processed at the 5′ terminus by RNase P, then the 3′ end is matured by an exo- and/or endo-nucleolytic pathway. The step of 3′ removal is very fast under standard conditions (indicated by a green arrow), but under stress it is inhibited (marked with a red dashed arrow) by limited amount/activity of Rex1 and increased protection by Lhp1 and other putative factors. Thus, 5′-end-processed 3′-end-extended precursors of tRNATyr can be detected under standard conditions and they accumulate to higher levels under stress. (B) tRNAUAUIle has a longer primary transcript than previously assumed (3′ extension marked with yellow) and it is removed in two steps. The first step is very fast under standard conditions (indicated by a green arrow) and the 3′ end is partially trimmed before the 5′ end is removed. Inhibition of this step by stress due to a relative excess of Lhp1 and other putative factors (marked with a red dashed arrow) results in accumulation of pre-tRNAUAUIle unprocessed at both 3′ and 5′ termini and part of these accumulated pre-tRNAs have longer 3′ extensions. The second step of 3′ end removal is done in concert with 5′ end cleavage, thus precursors processed at one end are not detected.
