High-resolution 300-MHz proton nuclear magnetic resonance spectra of the hydrogen-bonded protons in three different purified tRNA molecules are presented. The resonances in the region between -11 and -15 ppm from 2,2-dimethyl-2-silapentane-5-sulfonate (DSS) are assigned to the ring NH protons of specific base pairs by two approaches. First, intrinsic positions of -14.8 ppm and -13.7 ppm are taken for the AU and GC ring NH protons, respectively, and the spectra are calculated by including ring current shifts from the nearest neighbors.The spectra calculated in this way on the basis of the cloverleaf are in good agreement with the observed. Second, fragments of yeast tRNAPhe were obtained, which helped in assignments of the spectrum of intact molecules. The close agreement strongly supports the cloverleaf model.Tertiary structural features were determined in a few cases where the ring currents at the terminal base pairs of helical regions depended upon stacking of the helices.In this way, we were able to show that in Escherichia colt tRNAGlu the CCA stem forms a continuous helix with the TVt-C stem, which is in accord with the preliminary x-ray structure of yeast tRNAPhe, suggesting that this stacking is observed in solution and may be a general property of different tRNA molecules. Similar reasoning suggests that in E. coli tRNAfMet G-27 is stacked upon the dihydrouridine helix.In order to understand in detail how tRNA functions in protein synthesis, the structure of tRNA in solution must be determined. The present note summarizes results of our recent high-resolution proton nuclear magnetic resonance (NMR) investigations (1-4) of tRNA in which we determined that the cloverleaf model is an accurate description of the hydrogen-bonded secondary structure of several purified tRNAs, including yeast tRNAPhe, Escherichia coli tRNAGlU, and E. coli tRNAfMet. In addition, we obtained information about certain important tertiary structural features of each molecule. In this paper we emphasize these structural aspects to show how the NMR measurements in solution complement and supplement the crystal structure presently emerging from the x-ray studies (5).The groundwork for the present study was developed in previous studies of the NMR spectra of tRNA molecules in solution where we showed that resonances observed in the region between -11 and -15 ppm from 2,2-dimethyl-2-Abbreviations: NMR, nuclear magnetic resonance; DSS, 2,2-dimethyl-2-silapentane-5-sulfonate.

No abstract

The nuclear DNAs from a number of angiosperm species were tested for hybridization to the RNAs contained in 70 S (chloroplastic) and 80 S (cytoplasmic) ribosomes sequences of ribosomal RNAs have largely been conserved despite the evidence (1) that in other respects considerable differences can exist between DNAs of closely related species. Hybridization studies (32) have also clearly shown that the nuclear DNA from tobacco contains coding sequences for RNAs of both chloroplastic and cytoplasmic ribosomes.In this report, we present experiments which demonstrate that the extent of DNA coding for RNA of chloroplastic and cytoplasmic ribosomes can be estimated by hybridizing the DNAs with rRNAs from roots and leaves. With this method, DNAs from a number of closely and distantly related species were examined for their content of coding sequences for these RNAs. All plant nuclear DNAs thus far examined have been found to contain coding sequences for RNA of both chloroplastic and cytoplasmic ribosomes. METHODSRibosomes and labeled and unlabeled RNAs for all hybridizations were obtained primarily from roots and leaves of tobacco (Nicotiana tabacum L. var. Samsun). Tobacco seeds were germinated in vermiculite. When seedlings had a shoot length of about 6 inches, their roots were washed and the plants were transferred to 1-liter glazed pots containing continuously aerated complete Hoagland's medium (13), modified for rapid growth of cotton (D. R. Ergle, Department of Plant Sciences, Texas A and M University, College Station, Texas, personal communication). They were held for 3 to 4 weeks or until vigorous growth had begun. The complete medium was then replaced with -P medium for 2 days before adding I to 4 mc of carrier-free H3'2PO4

Ten Nicotiana species were assayed for the proportion of DNA that is complementary to ribosomal RNA. This proportion varies from 0.27 to 0.9 percent, with tetraploid species having lower values than the diploid species. The tetraploid species have about twice as much DNA per cell as do diploid species. Thus, the absolute number of genes for ribosomal RNA varies less than the proportion of complementary DNA. Further, the number of genes for the RNA in 80S ribosomes varies less among species than does that for the RNA in 70S ribosomes. The data indicate that loss of DNA complementary to ribosomal RNA is associated with tetraploidy in the genus Nicotiana.

employing a variety of experimental techniques i n d instrumentation. teachina limitations often exist. In particular, an Using Video Cassette Demonstrations in the Biochemistry Laboratoryexcessive amount i f instructor time is required to actually train each student to an acceotable level of competence and self-sufficiency in the use of each procedure. ~e i a u s e of the time constraints, students usually end up with incomplete learning, often attained through-watching or talkingwith other students. We have found that the use of demonstrations of techniques and of instrumentation, recorded on video cassettes significantly reduces these problems ( I ). This article describes (1) how the recorded demonstrations save considerable instructor and student time, (2) how such recordings of demonstrations mav he inteerated into a "multi media" teaching format, and (3) how recordings support student indenendence in studvine and in desienine exoeriments. . "A one-year, senior biochemistry lahoratory is often team taught and in such a course we found that effective teaching by many of the instructors was compromised by time limitations. For examole. manv orocedures are best tawht to m o u~s -.of only about fou; students. Use of small grou&, however, means that the demonstration must he reoeated several times. A number of repetitions of the same demonstration tend to "wear down" the instructor and thereby reduce his effectiveness. In addition, under any circums&ces, there are always students who reguest and who do benefit from repeated explanations of the p;ocedures or from review of thekxplanations a t later times. All of these situations dictate more instructor teaching time in the lahoratory hut much less information transfer than is possible by the instructor under other circumstances.Contact was made with educational media specialists at the campus Learning Resources Center to help solve this problem. I t was pointed out by them that lahoratory demonstrations could be recorded on videotapes inexpensively ($25 for a 60-min blank tape), in a brief time (about 2 hr per half hour of product) and that they could he replayed conveniently to groups of any size. These features were attractive to our fac-'Supported in part by NSF grant HES75-13043 ulty who want to maximize their teaching effectiveness during a relatively hrieinmract time with theclass. At the same time the taculr\ would only he amenable to using a recording medium which was simple, quick, and inexp&sive to use. We were encouraged when we were considering using video cassette recordings by the report of Pantaleo (2). He noted that unrestricted student use of video cassette recorded demonstrations increases their success in challeneine nrocedures bv -.20% in a freshman chemistry laboratory. This report suggeste& to us that the medium might also increase efficiency in teaching, among other techniques, gel electrophoresis, analytical ultracentrifugation, and scintillation spectrometry as found in a senior biochemistry laboratory.With assurances that we had located ...