The terminal phosphate groups of tobacco mosaic virus.

The nature of the end groups in various types of ribonucleic acids has been the object of several investigations. Work in this field has been hampered by the difficulty of isolating samples of ribonucleic acids under conditions which do not produce chemical changes in the molecule. In addition it appears probable that most ribonucleic acid preparations are polydisperse, with the exception of those derived from the simple viruses. The ribonucleic acid from tobacco mosaic virus (TMV-RNA)l is advantageous for these studies because it can easily be obtained by very gentle procedures and is believed to approach chemical and physical homogeneity. The early studies of Cohen and Stanley (1) indicated that the nucleic acid was large. More recent studies of the sedimentation behavior of TMV-RNA (2-4) have indicated a molecular weight of the order of 1 to 2 X 106. The ingenious electron microscopy studies of hot detergent-treated TMV carried out by Hart (5) also indicate that a particle of tobacco mosaic virus contains a single polynucleotide chain, the molecular weight of which would be 2.1 million on the basis of a particle weight of 40 million, containing 5.2% RNA (6). Consideration of the various possible methods for termination of the polynucleotide chain indicates that there are several possible structures for a ribonucleic acid molecule (see Fig. 1). The molecule may terminate in a 3’or, less probably, a 2’-phosphate (A), a 5’-phosphate (B), both a 3’and a 5’-phosphate (C), or no terminal phosphate (D), or it may terminate as a cyclic phosphate diester (E, F>. Other structures, e.g. branches from the 2’-hydroxyl group of the n-ribose moieties, are possible, but since no evidence exists for such structures, they will not be considered. In their pioneering studies, Markham, Matthews, and Smith (7) determined the number of nucleosides and nucleoside diphosphates which were formed upon alkaline digestion of TMV-RNA with and without prior treatment with prostatic phosphoesterase. Their results indicated that the molecules of TMV-RNA were on the order of 50 nucleotides long and, furthermore, that the virus nucleic acid consisted of a variety of types of molecules with the majority of Type B (Fig. 1). Matthews and Smith later reported that these results were due to contamination of the earlier virus preparations by low molecular weight polynucleotide material (8). These authors as well as Reddi and Knight (9) showed that there actually occurred less than one nucleoside 5’-phosphate end in 1000 residues, the limit of detectability by the methods used. Reddi and Knight favored the