To the mounting evidence of nonribosomal functions for ribosomal proteins, we now add L7Ae as a subunit of archaeal RNase P, a ribonucleoprotein (RNP) that catalyzes 5′-maturation of precursor tRNAs (pre-tRNAs). We first demonstrate that L7Ae coelutes with partially purified Methanococcus maripaludis (Mma) RNase P activity. After establishing in vitro reconstitution of the single RNA with four previously known protein subunits (POP5, RPP21, RPP29, and RPP30), we show that addition of L7Ae to this RNase P complex increases the optimal reaction temperature and k cat ∕K m (by ∼360-fold) for pre-tRNA cleavage to those observed with partially purified native Mma RNase P. We identify in the Mma RNase P RNA a putative kink-turn (K-turn), the structural motif recognized by L7Ae. The large stimulatory effect of Mma L7Ae on RNase P activity decreases to ≤4% of wild type upon mutating either the conserved nucleotides in this K-turn or amino acids in L7Ae shown to be essential for K-turn binding. The critical, multifunctional role of archaeal L7Ae in RNPs acting in tRNA processing (RNase P), RNA modification (H/ACA, C/D snoRNPs), and translation (ribosomes), especially by employing the same RNA-recognition surface, suggests coevolution of various translation-related functions, presumably to facilitate their coordinate regulation.pre-tRNA processing | RPP38 | protein-aided RNA catalysis R Nase P is a Mg 2þ -dependent endoribonuclease that is primarily responsible for catalyzing the removal of the 5′ leaders of precursor-tRNAs (pre-tRNAs) (1-3). Except for some unique organellar variants, RNase P functions in all three domains of life as a ribonucleoprotein (RNP) (1, 2). Although catalysis rests with the essential RNase P RNA (RPR) in all three domains of life (4-6), the RNase P protein (RPP) cofactors play essential roles. In the simple one RPR-one RPP bacterial RNase P, the RPP aids RPR catalysis by enhancing cleavage efficiency and affinity for substrate and Mg 2þ (7-9). The bacterial RPP has not been found in any archaeal or eukaryal genome (10). Eukaryal (nuclear) RNase P, which comprises an RPR and 9 or 10 RPPs (11, 12), has not been reconstituted from recombinant subunits, thus thwarting efforts to uncover the individual functions of eukaryal RPPs. Archaeal RNase P, with an RPR and four RPPs (all homologous to eukaryal RPPs), has therefore been explored as an experimental surrogate for its so-far-intractable eukaryal cousin (13-16). Although native archaeal RNase P has not been characterized, Western analysis and immunoprecipitation validated these four RPPs (POP5, RPP21, RPP29, and RPP30) as being associated with partially purified Methanothermobacter thermautotrophicus (Mth) RNase P activity (14). Subsequent structural and biochemical reconstitution studies using recombinant subunits have proven the utility of archaeal RNase P as a model system to dissect the role of multiple protein cofactors in facilitating RNA catalysis (16).Besides POP5, RPP21, RPP29, and RPP30, weak homologies are evident in the archaeal genomes...