David Baltimore, 1975 the Rockefeller University
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Rockefeller University Digital Commons @ RU Harvey Society Lectures 1976 David Baltimore, 1975 The Rockefeller University Follow this and additional works at: https://digitalcommons.rockefeller.edu/harvey-lectures Recommended Citation The Rockefeller University, "David Baltimore, 1975" (1976). Harvey Society Lectures. 51. https://digitalcommons.rockefeller.edu/harvey-lectures/51 This Book is brought to you for free and open access by Digital Commons @ RU. It has been accepted for inclusion in Harvey Society Lectures by an authorized administrator of Digital Commons @ RU. For more information, please contact [email protected]. THE STRATEGY OF RNA VIRUSES* DAVID BALTIMORE Department of Biology and Center for Cancer Research, Massachusetts Institute of Technology, Cambridge, Massachusetts I. INTRODUCTION N this presentation I shall contrast three types of animal viruses that I in their extracellular state appear very similar, but as replicating viruses are very different.The three types of viruses are: picornaviruses, typified by poliovirus; rhabdoviruses, typified by vesicular stomatitis virus (VSV), and RNA tumor viruses. The virions of all three of these viruses contain a single strand of RNA which is 2.5 to_ 4 X 106 daltons in length (on the order of 10 kilobases). While these viruses _might appear superficially similar because of the similarity of their genome structure, there are enormous differences in the modes of nucleic acid synthesis used by the viruses, the organiza tion of their messenger RNA (mRNA), the types of polymerases in volved in replicating them, and the way in which they utilize the hospitality of the host cell. Although many experimental observations have contributed to our understanding of these viruses, generally a single experiment provided the clue that signaled the unique features of a given virus. In what follows, the key observation for each of the systems will be discussed, and later work on the properties of the virus will then be summarized. The work from my laboratory described here was often performed by graduate students, postdoctoral fellows, or my close associate Donna Smoler. The names of my other principal associates are: for poliovirus work Michael Jacobson, Charles Cole, Deborah Spector, Martinez Hewlett, Lydia Villa-Komaroff, and Harvey Lodish; for VSV work, Alice Huang, Martha Stampfer, John Rose, David Knipe, and Harvey Lodish; and for RNA tumor virus work, Inder Verma, Amos Panet, and William Haseltine. *Lecture deliveredJanuary 16, 1975. 57 58 DAVID BALTIMORE II. POLIOVIRUS The key observation to our understanding of the life strategy used by the poliovirus genome was the observation many years ago that polio virus RNA is infectious (Colter et al., 1957; Alexander et al., 1958). At the time that observation was made, it merely confirmed what was known with tobacco mosaic virus, but the further development of animal virology has shown that only a few classes of viruses have infectiousRNAs. The existence of an infectious RNA implies that the RNA polymerase that replicates the RNA is either present in the uninfected cell or is specified directly by the incoming RNA acting as a mRNA. Because no polymerase able to replicate poliovirus RNA has ever been identified in uninfected cells, we have come to believe that at least a critical subunit of the poliovirus replicase is encoded by the viral RNA. A new replicase activity has been identified in the cytoplasm of poliovirus-infected cells (Baltimore et al., 1963)-this is the only evidence for a virus-specific subunit. Another approach to understanding the poliovirus strategy is to ask whether its virion RNA can act as mRNA. Before showing how we know that poliovirus RNA is a mRNA for the synthesis of the viral proteins, we first must introduce a variant of poliovirus called the poliovirus defectiveinterfering particle, or "polio DI" for short. Polio DI is a deletion mutant of poliovirus which appeared in our stocks as a contaminant and can be purified away from standard poliovirus because the DI virion has a lower density than the standard virion (Cole et al., 1971 ). The RNA of DI is smaller than standard RNA based on its migration rate during electrophoresis (Fig. 1), its sedi mentation rate in sucrose gradients and competitive hybridization studies (Cole and Baltimore, 1973). Poliovirus makes a double-stranded RNA also called "replicative form" or RF (Baltimore et al., 1964; Baltimore, 1966); an easy way to show that polio DI is a deletion is to measure the length of the standard and DI double-stranded RNAs. From electron micrographs of the double-stranded RNAs, histograms of the lengths of standard and DI RNA have been constructed (Fig. 2). They indicate a difference of 15-20% in the length of the two RNA molecules. Before turning to the question of whether virion RNA can act as THE STRATEGY OF RNA VIRUSES 59 80 300 60 . 200 -I C C .E E ...... ......"' "' 40 (.) (.) u ! if 100 20 30 40 50 65 Fraction Number FIG. 1. Comparison of the electrophoretic mobility of standard (- - -•) and DI (o--o) RNA in polyacrylamide gels. Standard poliovirion RNA labeled with 14 3 [ C] uridine was mixed with the RNA from DI particles labeled with [ H] uri dine. The two RNAs were analyzed by electrophoresis through 2.6% polyacryl amide gels. Migration was from left to right. Reprinted from Cole et al. (1971) by permission of the publisher. mRNA the peculiar nature of poliovirus-specific protein synthesis must be understood. While a large number of discrete polypeptides can be found in the cytoplasm of poliovirus-infected cells, evidence from various types of experiments points to the existence of only a single, long polypeptide (really a polyprotein) as the translation product of the poliovirus genome. This was initially demonstrated in a study of the sizes of the poliovirus proteins made when amino acid analogs replaced certain of the standard amino acids in the growth medium (Jacobson and Baltimore, 1968). A series of long polypeptides were found, the. longest of which was the appropriate size to be a total transcript of the poliovirus genome (Jacobson et al., 1970). We have more recently been able to prove that this is a transcript of the whole genome by looking at DAVID BALTIMORE 60 A Standard Poliovirus RF RNA 40 30 20 10 E 0 0 B DI Poliovirus RF RNA E 50 40 30 20 10 Length, µm FIG. 2. Comparison of the sizes of double-stranded RNA of (A) standard poliovirus and (B) DI particles. The lengths of poliovirus double-stranded RNA molecules (RF RNA) were determined by el ectron microscopy. Unpublished data of Dr. M. Hewlett. the size of this protein in cells infected by standard virus and in cells infected by polio DI (Cole and Baltimore, 1973). Figure 3 represents coelectrophoresis of the virus-specific proteins in cells infected by either standard virus or polio DI and incubated in the presence of amino acid analogs. The largest product (marked NCVP-00) is slightly shorter in the DI-infected cells ("DI(1) NCVP-00")than in the standard virus-infected cells, and calculations indicate that the loss of protein corresponds to about 15% of the total genome (Cole and Baltimore, 1973). Therefore the physical deletion of the RNA and the size of protein deleted are in agreement with each other and thus NCVP-00 would appear to be a translation product of virtually the entire polio virus RNA. To show that the poliovirion RNA is able to direct the synthesis of THE STRATEGY OF RNA VIRUSES 61 150.-----------------------� 0 � 100 0 a. > u u z -� E 50 0 0 40 60 80 100 120 Fraction number FIG. 3. Comparison of the electrophoretic mobility of virus-specific proteins induced by poliovirus and DI particles in the presence of amino acid analogs. Cells infected by standard poliovirus were labeled with [ 3 H)leucine (o--o); cells infected by DI particles were labeled with [ 14 C)leucine (---•). Both cultures of cells were exposed to medium in which the arginine, phenylalanine, praline, and methionine had been replaced by canavanine, p-fluorophenylalanine, azetidine-2- carboxylic acid, and ethionine. The proteins labeled in the two cultures were mixed and analyzed by electrophoresis through 7.2% polyacrylamide gels in the presence of sodium dodecyl sulfate. The protein "NCVP 00" coelectrophoresed with a marker of myosin indicating a molecular weight in excess of 200,000. Migration was from left to right. Reprinted from Cole and Baltimore (1973). this single, very long polypeptide, use was made of an in vitro cell-free system derived from mouse ascites cells. By shifting the salt concentra tion during a 90-minute incubation from the optimum for initiation (90 mM) to the optimum for elongation (150 mM), a significant amount of protein of the size of NCVP-00 has been made (Villa-Komaroff, 1975; Villa-Komaroff et al., 1975). When standard RNA was used to program the system, the longest polypeptide product had the same size as the NCVP-00 made in the presence of amino acid analogs (Fig. 4). When the system was programmed by polio DI RNA (Fig. 5), the size of the longest product was shorter than NCVP-00 by the same amount as the longest product made in cells infected by polio DI (compare to Fig. 3). Therefore the same products can be made in vitro using a system programmed by virion RNA as are made in vivo, and we can conclude that the virion RNA can act as mRNA. 62 DAVID BALTIMORE 0 0..0 > U O .Q 0.. Z O O u> I � � z u u � z z ii IJII I ,, ,, I � I w u I:,, : I I z II I' <( ,, 11.•i CD I I � ' I I a:: J I � •11 0 I l): I� (/) I 'A.I CD I <( I ,, I I•' I I I I J� \.