Extensive deproteinization of Dictyostelium discoideum RNase P reveals a new catalytic activity

Archaeal/eukaryal RNase P: subunits, functions and RNA diversification

RNase P, a catalytic ribonucleoprotein (RNP), is best known for its role in precursor tRNA processing. Recent discoveries have revealed that eukaryal RNase P is also required for transcription and processing of select non-coding RNAs, thus enmeshing RNase P in an intricate network of machineries required for gene expression. Moreover, the RNase P RNA seems to have been subject to gene duplication, selection and divergence to generate two new catalytic RNPs, RNase MRP and MRP-TERT, which perform novel functions encompassing cell cycle control and stem cell biology. We present new evidence and perspectives on the functional diversification of the RNase P RNA to highlight it as a paradigm for the evolutionary plasticity that underlies the extant broad repertoire of catalytic and unexpected regulatory roles played by RNA-driven RNPs.

Hybrid E. coli--Mitochondrial ribonuclease P RNAs are catalytically active

RNase P is a ribonucleoprotein that cleaves tRNA precursors at their 5'-end. Mitochondrion-encoded RNA subunits of mitochondrial RNase P (mtP-RNA) have been identified in jakobid flagellates such as Reclinomonas americana, in the prasinophyte alga Nephroselmis olivacea, and in several ascomycete and zygomycete fungi. While the structures of ascomycete mtP-RNAs are highly reduced, those of jakobids, prasinophytes, and zygomycetes retain most conserved features of their bacterial counterparts. Therefore, these mtP-RNAs might be active in vitro in the absence of a protein subunit, as are bacterial P-RNAs. Here we present a comparative structural analysis including seven newly characterized jakobid mtP-RNAs. We investigate ribozyme activities of mtP-RNAs and find that even the most bacteria-like molecules of jakobids are inactive in vitro. However, when certain domains of jakobid and N. olivacea mtP-RNAs are replaced with those from Escherichia coli, these hybrid RNAs show catalytic activity. In vitro mutagenesis of these hybrid mtP-RNAs shows that various structural elements play a critical role in ribozyme catalysis and provide further support for the presence of these elements in mtP-RNAs. These include GNRA tetraloops in helix P14 and P18 of Jakoba libera, and a remnant P3 pairing in Seculamonas ecuadoriensis. Finally, we will discuss reasons for the failure of mtP-RNAs to show catalytic activity in the absence of P-proteins based on our mutagenesis analysis.

Aminoglycosides suppress tRNA processing in human epidermal keratinocytes in vitro

The ever-growing resistance of pathogens to antibiotics and the lack of potent antibacterial drugs constitute major problems in the treatment of infectious diseases. Thus, the better understanding of the mode of action of antibiotics at the molecular level is of essential importance. Accumulating evidence points towards RNA as being a crucial target of antibacterial and antiviral drugs. Interestingly, aminoglycosides, one of the most important families of antibiotics, apart from their inhibitory effect on ribosome function, reportedly interfere with various RNA molecules and in vitro suppress the proliferation of human keratinocytes. In this study we investigated the effect of the aminoglycosides neomycin B, paromomycin, tobramycin and gentamycin on ribonuclease P activity from normal human epidermal keratinocytes. All aminoglycosides tested revealed a dose-dependent inhibition of tRNA maturation, which was reduced by increasing Mg(2+) ion concentrations, indicating competition of the cationic aminoglycosides with magnesium ions required for catalysis. Our in vitro findings suggest that the inhibitory effects of aminoglycosides on tRNA processing may be implicated in the mechanisms of their antiproliferative action on human epidermal keratinocytes.

Kinetics of inhibition of ribonuclease P activity by peptidyltransferase inhibitors. Effect of antibiotics on RNase P

A cell-free system derived from Dictyostelium discoideum has been used to study the kinetics of inhibition of RNase P by puromycin, amicetin and blasticidin S. Detailed kinetic analysis showed that the type of inhibition of RNase P activity by puromycin is simple competitive, whereas the type of inhibition by amicetin and blasticidin S is simple non-competitive. On the basis of Ki values amicetin is stronger inhibitor than puromycin and blasticidin S.

Retinoids inhibit human epidermal keratinocyte RNase P activity

Ribonuclease P (RNase P) is a ubiquitous and essential enzyme that endonucleolytically cleaves all tRNA precursors to produce the mature 5'-end. We have investigated the effect of synthetic rertinoids (all-trans retinoic acid, acitretin) and arotinoids (Ro 13-7410, Ro 15-0778, Ro, 13-6298 and Ro 15-1570) on RNase P activity isolated for the first time from normal human epidermal keratinocytes (NHEK). All tested compounds but one (Ro 15-1570) revealed a dose-dependent inhibition of RNase P activity, indicating that they may have a direct effect on tRNA biogenesis. Detailed kinetic analysis showed that all retinoids behave as classic competitive inhibitors. On the basis of the Ki values Ro 13-7410 was found to be the strongest inhibitor among all compounds tested.