RNase MRP is a ribonucleoprotein endoribonuclease that has been shown to cleave mitochondrial primer RNA sequences from a variety of sources. The bulk of RNase MRP activity is found in the nucleus where its function remains unknown. Two different approaches have resulted in predictions of distinct secondary structures for RNase MRP RNA. In order to analyze more definitively the higher-order structure of RNase MRP RNA, we have conducted a phylogenetic comparison of the available RNase MRP RNA sequences from human, mouse, rat, cow, toad, and yeast. The resulting secondary structure shares features in common with previously described structures for prokaryotic and eukaryotic RNase P RNAs (1) and RNase MRP RNAs (2, 3). In addition, the phylogenetic structure is consistent with available chemical modification data on RNase MRP RNA and with the detailed analysis of the To antigen binding domain located near the 5' end of the RNase MRP RNA. The structure is not limited to RNase MRP RNAs, but can be expanded to cover both eukaryotic RNase P RNAs and RNase P/MRP RNAs from plants.

Many previous workers have found low intensity ultraviolet (UV) radiation less effective than high intensity UV in producing injury to various living organisms (Christensen, 1953;Coblentz and Fulton, 1924;Dreyer, 1903;Gates, 1929; Swann and del Rosario, 1932;Weinstein, 1930;Wyckoff, 1932). Experiments performed here on the protozoan Didinium nasutum, indicated an opposite, greater effect of low intensity UV. It appeared likely that a dark reaction succeeds the absorption of UV, so that quanta are supplied at high intensity more rapidly than they can be utilized; i.e., saturation occurs. To investigate this possibility the UV was flashed and the dark period between flashes varied, with the expectation of increased injury with longer dark periods. Since a dark reaction is thermochemical, it should proceed more rapidly at high than at low temperature, and injury should therefore increase with temperature. Experiments were accordingly carried out at various temperatures. The expectations have been confirmed in the present study. The effects of variable light period and flash rate, and the comparison of flashed high intensity with continuous low intensity irradiation have also been explored. Materials and MethodsCultures of Didinium nasutum were grown in 4 ram. bore isolation tubes on concentrated suspensions of Paramecium caudatum in the manner previously described (Brandt el a/., 1955). Didinia selected at random for a sample were irradiated in a quartz cell, put into isolation tubes with food, one animal per tube, and the number ha each tube was recorded three times daily, as a rule, until the fourth division was reached ha all tubes. This stage was usually reached earlier in some tubes than ha others, and the tubes were discarded as they attained it, since higher counts were difficult to make, and were likely to be biased because of rapid depletion of the food supply.