RESEARCH
tRNA nucleotide sequence deciphered Carriers of amino acids in genetic code translation have common structural and steric properties The nucleotide sequence of phenylalanine transfer ribonucleic acid, phetRNA, has been deciphered by Dr. U. L. RajBhandary, Dr. H. G. Khorana, and their coworkers at the University of Wisconsin [Proc. Natl. Acad. Sci. U.S., 57, 751 (1967)]. This tRNA can adopt a secondary, cloverleaf structure similar to that of other
tRNA's of known structure, and is the latest of a number of tRNA's whose structures have been elucidated by many scientists the world over. And it's another example of a growing number of scientists using more tools in what is an increasing effort to dissect the molecules that carry amino acids to the ribosomes.
COLUMN CHROMATOGRAPHY. Dr. U. L. RajBhandary (rear, right) and Dr. S. H. Chang look on while Doug Davies and Judy Sneider operate a column used for chromatography of nucleotide fragments. Longer columns were used to resolve fragments of phe-tRNA 46 C&EN JUNE 5, 1967
Since the nucleotide sequences of several tRNA's have been determined and the study of several others is under way, the trend may be to determine the functional sites of these molecules. Many investigators are involved in modifying these molecules (chemically, physically, and enzymically) to see what functional changes result from structural modification. tRNA's most important function is to carry amino acids in the translation of the genetic code. A trinucleotide codon (a specific sequence of amino acids in the messenger RNA molecule) specifies a certain amino acid— phenylalanine, for instance—because this codon recognizes a suitably placed trinucleotide sequence that is present only in a species of tRNA capable of carrying phenylalanine. There are other reasons for the increasing interest in tRNA's. They are the smallest of all ribonucleic acids in that they contain between 70 and 90 nucleotide units. For example, phe-tRNA contains 76 units. This compares to other RNA's which contain hundreds and even thousands of nucleotide units. Each unit contains a purine base (adenine or guanine) or a pyrimidine base (uracil or cytosine), the sugar ribose, and a phosphate group. Also, and perhaps more important, the tRNA's can now be separated easily from the other cell constituents. Before anyone could seriously think of determining a sequence of one of these tRNA's, it had to be isolated in high purity and good yield. The application of countercurrent distribution techniques to separate and purify tRNA's has been a prime factor in accelerating structural work. With these techniques, many investigators are able to obtain good quantities of purified tRNA's. (Countercurrent distribution techniques are also used to separate other biological materials, including proteins.) Once the molecules are obtained in pure form, they must be selectively degraded to fragments (oligonucleotides ). Then, each fragment must be separated and purified, and the nucleotide sequence in the fragment determined. Finally, the whole sequence must be pieced together like a jigsaw puzzle.
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