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J. gen. Virol. (1969), $, 379-389 379 Printed in Great Britain Correlations between the A m i n o A c i d and N u c l e o t i d e C o m p o s i t i o n o f Plant Virus Particles: Evidence that Plants use the same Genetic C o d e as Bacteria BY A. J. G I B B S Department of Microbiology, John Curtin School of Medical Research, Australian National University, Canberra, Australia AND G. A. M A c l N T Y R E Division of Mathematical Statistics, Commonwealth Scientific and Industrial Research Organization, Canberra, Australia (Accepted IO June 1969) SUMMARY Correlations between the amino acid and nucleotide compositions of the particles of 41 plant viruses suggest that plant viruses, and hence presumably plants, use the same genetic code as bacteria, and that the gene for the protein in the particles of each of these viruses has a nucleotide composition similar to that of the whole nucleic acid molecule of the virus. INTRODUCTION It is now widely accepted that when a protein is synthesized in a cell the sequence of amino acids in the protein is determined by the sequence of bases in a messenger ribonucleic acid; each amino acid is specified by a sequence of three nucleotides called a codon. Each amino acid is specified by one or more codons. Experiments to determine directly which codons specify each amino acid have been done in various ways using bacterial cell extracts and viruses (summarized by Sadgopal, 1968). This assignment of codons we will call the bacterial genetic code. Direct experiments, similar to those with bacteria, to see whether plants use the same code have not been satisfactory for various technical reasons, but three types of indirect evidence suggest that plants do use the same code. All come from work with viruses, and assume that the viruses use the host's mechanisms for translating the information in their nucleic acids into proteins, and hence use the same code. The most convincing evidence is from studies of artificially induced mutants of tobacco mosaic virus (TMV) (evidence summarized by Fraenkel-Conrat, I968). The amino acid changes induced in the protein of the particles of this virus, when the virus nucleic acid has been treated with nitrous acid, are mostly consistent with the bacterial genetic code; nitrous acid deaminates adenine and cytosine to form hypoxanthine and uracil respectively. However, even if we accept that nitrous acid only deaminates the bases in TMV nucleic acid (and there is no evidence for this), these studies do not Downloaded from www.microbiologyresearch.org by IP: 88.99.165.207 On: Sun, 18 Jun 2017 19:30:55 380 A. J. GIBBS AND G. A. MACINTYRE exclude the possibility that the plant code is partially or wholly a mirror image of the bacterial code (i.e. the roles of adenine and cytosine, and of guanine and uracil are reversed), nor do they explain why several of the TMV mutants have amino acid changes that are not compatible with the bacterial genetic code. The other evidence that suggests that plants use the bacterial genetic code is much less convincing; there are reports (Clark et al. I965; van Ravenswaay-Claasen et aL I967) that some plant virus nucleic acids, when put into Escherischia coli cell-free protein-producing systems (Nirenberg & Matthaei, I960, induce the formation of proteins like those produced in the host plant. These experiments have not yet been confirmed, and similar experiments with several other plant virus nucleic acids have failed. There are also a few plant viruses that multiply in their insect vectors, but it is not known whether all the nucleic acid of these viruses is translated into proteins in both plant and insect; different parts of the nucleic acid of these viruses may be specially adapted to each type of host. Finally, claims have been made that a bacteriophage will grow in plants (Sander, I964; Schwartz et aL I965), and a plant virus in animal cells (Atherton, I968), but neither these nor similar experiments with bacterial and animal cells and viruses have been confirmed yet. In this paper we describe an alternative, though still indirect, way of checking whether the plant and bacterial genetic codes are similar. The nucleic acids and proteins of the particles of many different plant viruses have now been analysed chemically, so that one can test whether the amino acid composition of the proteins of these viruses is what one would expect from the nucleotide composition of their nucleic acids if the code used in the translation mechanism in plants is similar to that used by bacteria. To test this idea we must assume that the gene for the protein of the particles of each plant virus has a composition similar to that of the whole nucleic acid molecule of the virus, and that plants use the several codons associated with an amino acid equally frequently. What evidence there is suggests that the first assumption may be correct, for when the nucleic acids from different components of a multicomponent plant virus, or from defined fragments of a plant virus nucleic acid, have been analysed, most have been found to have an almost identical base composition (evidence summarized by Gibbs, I969). There is some evidence to suggest that the second assumption may be partly incorrect (Streisinger et aL 1966; Inouye et al. I967), and codons ending in guanine or cytosine may be used infrequently in bacteria; therefore in one test we compared the full bacterial code with an 'A/U-rich' code, omitting, where possible, all codons ending in guanine or cytosine. We have tested our idea in two ways. We computed correlation coefficients between the nucleotide and the amino acid compositions of the particles of 41 plant viruses, to see whether the correlations obtained are related to the codon assignments for amino acids in the bacterial genetic code. We also used the bacterial genetic code to predict the amino acid composition of the particles of these viruses from their nucleotide composition, and vice versa, and then tested whether the predicted and observed compositions are correlated. Sources of information The viruses, and the sources of the results of analyses that we used, are given in Table I. Virus names and cryptograms are from Martyn (I968). Three-letter abbreviations are used in the tabulated results for the names of amino acids (Dayhoff & Eck, Downloaded from www.microbiologyresearch.org by IP: 88.99.165.207 On: Sun, 18 Jun 2017 19:30:55 Amino acid and nucleotide composition 381 1968). Not all the viruses had been analysed for cysteine and tryptophan, therefore these were omitted from the analyses. The average nucleotide composition of the codons for each amino acid (Table 2) was calculated from the bacterial codon assignments tabulated by Sadgopal 0968), assuming, except where stated, that the several codons for each amino acid were used equally frequently. For one test the average nucleotide composition of codons for each amino acid was calculated from, where possible, only those codons in which adenine or uracil is the third base of the triplet. T a b l e I. The viruses, and the sources of information Alfalfa mosaic virus; R/x:x.3/x8:U/U:S/Ap. (top and bottom components). Kelley & Kaesberg (I962); Rauws, Jaspers & Veldstra (I964). Bean pod mottle virus; R/x:2"4/35:S/S:S/CI; Cowpea mosaic virus group. Semancik & Bancroft (1964); Semancik 0966). Bean southern mosaic virus; R/x:x-4/2x :S/S:S/Ap. (cowpea, Mexican and type strains). Ghabrial, Shepherd & Grogan (1967). Broad bean mottle virus; R/x : x'x/22:S/S:S/*; brome mosaic virus group. Aronson & Bancroft (1962); Yamazaki & Kaesberg (1963). Brome mosaic virus; R/x: x/22: S/S: S/*; brome mosaic virus group. Bockstahler & Kaesberg (i965); Stubbs & Kaesberg (1964). Carnation mottle virus; R/x:*/*:S/S:S/*. R. MacLeod (personal communication). Carnation ring spot virus; R/x : x'4/2o: S/S :S/*. Kalmakoff & Tremalne (1967). Clover (white) mosaic virus; R/x: */5: E/E: S/(Ap); potato virus X group. Fry, Grogan & Lyttleton (196o); Miki & Knight (1967). Crotalaria (Sann Hemp) mosaic virus; R/x:*/* :E/E:S/*; tobacco mosaic virus group. Rees & Short (I965); R. H. Symons (personal communication). Cucumber green mottle mosaic virus; R/x:*/* :E/E:S/*; tobacco mosaic virus group. Knight (1952); Markham & Smith (195o); van Regeumortel (I967b). Cowpea chlorotic mottle virus; R/x: x.x/z4: S/S: S/*; brome mosaic virus group. Bancroft et aL (1968). Cucumber mosaic virus; R/x:x/I8:S/S:S/Ap. Francki et al. (1966); Kaper, Diener & Scott (1965); van Regenmortel (I967a). Cucumber (wild) mosaic virus; R/I:2"4/35:SIS:S/CI; turnip yellow mosaic group. Symons et aL (I963). Echtes Ackerbohnenmosaik virus; R/x : "1" : SIS: S/*; cowpea mosaic virus group. Gibbs, GiussaniBelli & Smith (1968); Wittmann & Paul (I961). Pea enation mosaic virus; R/x:x'3127:S/S:S[Ap. Shepherd, Wakeman & Ghabrial (1968). Pelargonium leaf curl virus; R/x:x'5/x7:S/S:S/*. Dorner & Knight (I953); de Fremery & Knight (z955). Potato virus X; R/x: */6:E/E:S/(Fu); potato virus Xgroup. Dorner & Knight (I953); Miki & Knight, (1968); Reichmann (1964). Satellite virus; R/x:o.4/2o:S/S:S/Fu. Reichmann (I964). Sowbane mosaic virus; R/x: x'3/x7: S/S :S/Di. Kado (1967). Squash mosaic virus; R/x:2"4/35:S/S:S/CI; cowpea mosaic virus group. Mazzone, Incardona & Kaesberg (1962). Tobacco mosaic virus; R/x :2/5:S/S:S[*; tobacco mosaic virus group. (GA, J I4DI, masked, ribgrass, type, YA, and Y-TAMV strains) Domer & Knight (1953); Knight (I952); Markham & Smith (I95o); Tsugita (1962); Wang & Knight (I967). Tobacco rattle virus; R/x:2"3/s:E/E:S/Ne; Netu virus group. Offord & Harris (1965); Semancik & Kajiyama 0967). Tobacco ringspot virus; R/x: x.8142: S/S: S/Ne; Nepo virus group. Randles & Francki (1965); StaceSmith, Reichmann & Wright (1965). Tomato ringspot virus; R/x:*/4I:S/S:S/Ne; Nepo virus group. Tremaine & Stace-Smith (1968). Turnip crinkle virus; R/x :2/25:S/S:S/C1. Symons et al. (1963); R. MacLeod (personal communication). Turnip yellow mosaic virus; R/I:X'9/37:S/S:S/CI; turnip yellow mosaic virus group (cauliflower, Denmark, Honesty, Rademacher, Rothamsted and type strains). Symons et aL (1963). Downloaded from www.microbiologyresearch.org by IP: 88.99.165.207 On: Sun, 18 Jun 2017 19:30:55 A. J. G I B B S A N D G. A. M A C I N T Y R E 382 Table 2. Correlations between the nucleotide and amino acid composition of the particles of 41 plant viruses, and their agreement with the bacterial code Composition of plant virus particles Mean of Amino acid particles ala arg asx glx gly his ile leu lys met phe pro ser thr tyr val 9"3 5-2 9"5 8-3 6-I 1-2 5"2 8"5 4"0 1-6 4"0 6"3 9"0 9"3 2-6 7"7 ~ Correlations* of the amino acid composition with nucleotides ~ ~~ G A C U o.2o 0"67 0.28 o'37 0.28 --0"4I --O'7O --0"30 o'I2 --0-22 0-24 --o'79 -0"25 --0"29 o'56 0"35 0"34 0"27 0"43 o'40 0-03 --0"52 --0"29 --0'58 0-06 -0"08 0"37 -o.6I --o'I2 0"05 o'o4 o'o3 -0"23 -0'57 -0-46 -0"46 -0-27 0"51 o-60 0"38 --0"07 o'23 --0"52 o'87 o'o9 0'3 ° -o'41 --o'I4 -0.06 0"30 0"36 0-24 o'31 --0"I9 --O'34 0"07 o-lo -o"I9 0'60 -o'57 o'17 -0"46 0-3o -o'Io o'71 -o.12 -o'I5 -o-II 0'45 0"49 -0"56 -0"33 0.22 0"27 0"62 o-19 -0"43 -0"25 0"05 0"33 -o'13 -o"3I -0"07 0-26 -0"77 -0"03 o'29 0"03 -0-38 -o'55 0-52 o.28 -o.2o -0.42 0'38 o-03 -0"o7 0'33 0"37 0"43 -0"50 0"06 0"30 0-45 -0'08 -o-24 o'24 -o'25 Bacterial genetic code. Mean nucleotide compositiont of codons for each amino acid r ~ ~ G A C U 0"42 0'44 o'17 0"33 0"75 O'00 O'00 o'11 oq7 0"33 o.oo o'o8 o'I7 0.08 o'oo 0-42 Agreement~ , ~--~ Measure of Agree- Relative ment Value 0.08 0.22 0"50 0"5° 0.08 0"33 O'44 Oql 0"83 0'33 o.oo o.o8 o'17 0"42 o'33 0.08 0"42 0.28 o'I7 o'17 0-08 0"50 O.II o'27 o-oo o'oo o'17 o'75 o'33 0"42 o'17 0-08 0.08 0-06 o'I7 o.oo 0.08 O'I7 o'44 0"50 o'oo 0'33 0"83 o'o8 o'33 0-08 o'5o 0-42 -0"35 o.i8 0"44 0-4° 0"47 0.68 --o-o8 0"63 o'17 --0'95 o'4I o'99 o'95 0"93 -o.H o'46 0.86 0"39 0"74 0'84 0"97 O'I2 0"56 0"74 0"75 o'I2 0"75 I.OO o'39 0"86 o'I 4 0-72 0'50 o-I7 o"17 o"15 o'5I oq7 o'I7 o't7 o"19 0"07 oq7 0"50 o-17 o.Io o.16 o'50 o'I7 o'17 0-36 0"33 o'17 o'I7 0'50 o'21 o'18 o't9 0"5o o't7 o't9 0"23 oq7 o-I7 o'I7 0'54 o'16 o"I4 o'I7 0'50 0"25 0"37 0"46 0-88 0"85 0"97 0"54 o-I2 0'92 0"30 -o'14 0-22 0"93 0"95 0"66 0'60 0"48 0'84 I'OO 0"87 0.22 0"45 0"30 o.16 o'32 0.22 0"83 0"89 Grouped amino acids§ GI 28-8 AI 29"4 C1 2o'4 UI I5"4 G2 14"3 A2 25-6 C2 30"9 U2 27"0 GA3 13"9 CV3 17"3 GACU 3 61"4 * Correlation coefficients; levels of significance P o'oI = o'39; P o'ooI = 0'49. t Average codon composition calculated from Sadgopal (I968). :~ For explanation, see text. § Amino acids grouped according to bacterial genetic code by the first, second and third base in their codons Group G i contains ala, asx/2, glx/2, gly, val. A t arg/3, asx/2, ile, lys, met, set/3, thr. C1 arg2/3, glx/2, his, pro, leu2/3. U I leu/3, phe, set2/3, tyr. G2 arg, gly, ser/3. A2 asx, glx, his, lys, tyr. C2 ala, pro, ser2/3, thr. U 2 ile, leu, met, phe, val. GA3 contains those amino acids whose codons end in either G or A, that is glx, lys, and met. CU3 contains those whose codons end in either C or U, that is asx, his, phe, and tyr. G A C U 3 contains those whose codons end in either G, A, C or U and include all the remaining nine amino acids. RESULTS Correlations Correlation coefficients were calculated between the amounts of each of the four nucleotides and sixteen amino acids in the particles of the 4I viruses, and also between each of the nucleotides and the amino acids grouped in various ways. The amino acids were grouped according to which base occurred first in the codons assigned to the amino acids in the bacterial genetic code, which occurred second, and which third. Downloaded from www.microbiologyresearch.org by IP: 88.99.165.207 On: Sun, 18 Jun 2017 19:30:55 Amino acid and nucleotide composition 383 Most of the correlations between the nucleotide and amino acid composition of these virus particles (Table z) are closely related to the codon assignments in the bacterial genetic code. For example, arginine is closely positively correlated with guanine, and negatively or less correlated with the other nucleotides. This agrees well with the bacterial genetic code, in which the arginine codons have an average composition of G o.44:A o.zz:C o.z8:U 0.06. The correlations for nine of the 16 amino acids agree with the bacterial code, six closely. With the grouped amino acids there is good agreement between the bacterial code and the correlation coefficients for those grouped according to the first nucleotide in the codon, but less agreement with those grouped according to the second and third nucleotides in the codon. To show the extent of the agreement more clearly, a measure of agreement (correlation coefficient) was calculated for each amino acid (or group of amino acids) between the average nucleotide composition of the corresponding bacteriaI codons on the one hand, and the correlation coefficients between the amino acid and nucleotide composition of the plant viruses on the other. These measures of agreement (Table 2, penultimate column) were positive for all except four of the ungrouped amino acids and one of the grouped amino acids. Obviously the possibility of getting any agreement will be greater with those amino acids that form the bulk of the virus protein than with those that are present in small amounts, and will also be greater with those amino acids whose bacterial codons have greatly different amounts of the four nucleotides than with those that have similar amounts of the four nucleotides. Therefore we calculated the relative value (Table 2, last column) that can be attached to each measure of agreement; the relative value is arbitrarily defined for each amino acid (or group of amino acids) as the standard deviation of the mean amounts of individual nucleotides in the codons for that amino acid multiplied by the mean percentage of that amino acid in the virus particles, and is expressed relative to the 'value' for proline (to C2 for the grouped amino acids). Five negative measures of agreement were obtained; these were for alanine, isoleucine and three other amino acids with such small relative values, that they may be disregarded (Table 2). There is no obvious explanation for the result obtained with alanine, but that for isoleucine may reflect the fact that there are correlations between the amounts of the four nucleotides (Table 3) and between the amounts of the different amino acids (Table 4) in the virus particles. The isoleucine content of the viruses is correlated with their cytosine content, yet it would be expected from the bacterial code to be correlated with their adenine and uracil content (Table z). Isoleucine, which constitutes on average 5"2 ~o of the virus proteins, is positively correlated with both proline and threonine, which together constitute 15"6 ~o of the proteins (Table 4), and which are both positively correlated with cytosine (Table z). Thus it is not surprising that isoleucine is also positively correlated with cytosine. The correlations between the nucleotides in the nucleic acids of these viruses (Table 3) presumably reflect the fact that there are only four nucleotides and their amounts were expressed for these calculations as percentages. Thus cytosine, which has the largest range (from I6.2 ~ to 42-I ~o of the nucleic acid), is inevitably negatively correlated with the other three nucleotides. The amino acids were also expressed as percentages, but, as there are sixteen of them, the correlations between them (Table 4) are not just the result of a mathematical constraint, and perhaps reflect differences between the viruses in their use of the different amino acids in their sub- Downloaded from www.microbiologyresearch.org by IP: 88.99.165.207 On: Sun, 18 Jun 2017 19:30:55 384 A.J. GIBBS AND G. A. MACINTYRE units, all of which are similar globular protein molecules. Amino acids may be grouped in various ways according to the properties of their side chains (summarized by Zimmerman, Eliezer & Simha, r968 ). Thus the negative correlations between methionine and phenylalanine are perhaps because these amino acids are of similar size and polarity and hence serve similar functions in the protein. Other correlations may reflect the ionization properties of the amino acids rather than their size; thus the basic amino acids arginine and histidine are negatively correlated, whereas arginine is positively correlated with glutamic acid (and amine). Table 3. Correlations* between the amountst of different nucleotides in the nucleic acids of 4I plant viruses G 0"34 A - 0"79 -0"73 C 0-40 0.30 -0"73 U * Correlation coefficients; significance levels as in Table 2. t Amounts expressed as moles per cent. Table 4. Correlations* between the amountst of different amino acids in the particle proteins of 4I plant viruses ala arg 0"41 asp glu • --o"41 -0'44 -0'65 o'43 --o'4I . 0"48 -0"42 gly • -o"47 his . fie o'65 0"40 o'44 leu -o'4z lys - 0"50 - 0-46 0"44 o-52 o'44 -o'4~ -0"55 met - 0"44 phe pro --o'41 ser tier tyr val * Correlation coefficients; significance levels as in Table z. For simplicity only statistically signiflcant correlations are listed. t Moles %. Predicted compositions The amino acid composition of each virus was predicted indirectly from its observed nucleotide composition (Gobs, Aobs, Cobs, Uob,), using the average bacterial codon composition (Godn, Aodn, C~n, Uodn) (Table 2), to calculate for each amino acid in turn, the sum Gobs. Godn+ Aob,. Aodn+ Cob~.Co~ + Uob,. Uodn. The nucleotide composition of each virus was predicted from its amino acid composition in a similar way Gpr,d = 2 ~ X . Godn; Aped = 27~ X.Aod~; etc., where X is the amount of each amino acid ( a l a . . . val) in the virus protein. Downloaded from www.microbiologyresearch.org by IP: 88.99.165.207 On: Sun, 18 Jun 2017 19:30:55 Amino acid and nucleotide composition 385 Correlation coefficients calculated to compare the predicted and observed nucleotide and amino acid compositions (Table 5) show that all four nucleotides and seven of the 16 amino acids were successfully predicted. It is surprising that the predicted nucleotide composition of the genes for the protein in the particles is close to the observed composition of the whole nucleic acids and suggests that the nucleic acid molecules of these viruses are relatively homogeneous in composition, and in this respect differ from those of the larger bacteriophages (Hogness & Simmons, 1964). Table 5. Correlations* between observed and predictedt (a) nucleotide composition and (b) amino acid composition of the particles of 4I plant viruses using full bacterial code * (a) Guanine Adenine Cytosine Uracil (b) 0'7 I 0'31 0"74 0'33 ala arg asx glx gly his ile leu o"19 o'z2 0"44 0"42 o.28 0"47 0"24 o'61 lys met phe pro ser thr tyr val 0'03 0.22 0"47 0'87 0.22 0"44 o-or o"18 * Correlations coefficients; levels of significanceP 0"05 = 0"30; P o-oI 0"39; P o-oot = 0"49. i" Method for predicting composition using bacterial genetic code described in text. :~All codons in the bacterial code were used to calculate the average base composition of the codons of each amino acid. = The predicted average guanine and cytosine contents of the viruses are slightly greater than the observed amounts, and the adenine and uracil contents less; the predicted average base ratio was G 24"4:A 26.2:C 25"8:U 23"4, whereas the observed ratio was G 23"4:A 26"3:C 24"4:U 25"8. The regression coefficients for the relation between the predicted and observed amounts of the bases also varied; G I ' I 7 : A o'79:C 3"4o:U 0"67. These results suggest that if all possible codons are used by plants equally frequently, then the gene for the protein in the particle of these viruses has a higher guanine and cytosine content than other parts of their nucleic acids. Alternatively if that gene has the same composition as the whole nucleic acid molecule of the virus, these results suggest that the whole nucleic acid uses more of t h e ' adenineuracil' rich codons than the 'guanine-cytosine' rich codons for each amino acid. The latter possibility would agree with the findings of Streisinger and his colleagues (Streisinger et al. I966; Inouye et al. 1967), who determined the precise composition of six codons of bacteriophage T 4. All the codons they found had adenine or uracil as third base in the codon, suggesting that codons ending with guanine or cytosine are rare or absent. To test whether our data would show that plant viruses have a similarly restricted code we calculated the average nucleotide composition of the codons of each amino acid, omitting where possible all ending with guanine or cytosine. Then we used the 'A/U-rich' codon composition to predict, as before, the amino acid composition of the viruses from their nucleic acid composition, and vice versa, and calculated correlation coefficients between the predicted and observed compositions (Table 6). The 'A/U-rich' code did not predict the nucleotide composition of the viruses as well as the complete code; the average ' g u a n i n e + c y t o s i n e ' content of the viruses when predicted by the restricted code was 34.2 70 but when 25 J. Virol. 5 Downloaded from www.microbiologyresearch.org by IP: 88.99.165.207 On: Sun, 18 Jun 2017 19:30:55 386 A . J . GIBBS AND G. A. MACINTYRE predicted by the full code was 50"2 ~o, compared with the observed average content of 47"8 ~ . However the prediction of amino acids was better with the 'A/U-rich' code than with the full code; the correlation coefficients for nine of the amino acids increased, only four decreased. Table 6. Correlations* between observed and predicted~ (a) nucleotide composition and (b) amino acid composition of the particles of 4I plant viruses, using 'A/U-rich' code~ (a) (b) Guanine o'69 Adenine Cytosine Uracil 0"29 0"76 0"29 ala arg asx glx gly his ile leu o. t 9 0"47 0"49 o'35 o'29 0"42 o'39 o'3o lys met phe pro ser thr tyr val 0-06 0-22 0'60 o'88 o'33 o'45 o'25 o. I I * Correlation coefficients; levels of significance P 0'05 = 0"30; P o'oI 0"39; P o,ooi t Method for predicting composition using bacterial genetic code is described in text. = = o'49. :~The average base composition of the bacterial codons of each amino acid was calculated omitting, where possible, all codons with guanine or cytosine as third nucleotide in the codon. DISCUSSION The results given above indirectly indicate that plants use a genetic code broadly similar to that used by bacteria, and exclude the possibility that the genetic code used by plants is a mirror-image of that used by bacteria. All the codons may not be used equally frequently by plants, and it is possible that for some amino acids those with adenine or uracil as the third base in the codon are used more frequently than those with guanine or cytosine in that position. It is interesting that the correlations we have found agree closely with results obtained by Sueoka (I 96 I) with bacteria. Sueoka analysed the amino acid composition of various bacteria with nucleic acids of different nucleotide composition, and found that the guanine + cytosine content of the bacteria was positively correlated with their content of alanine, arginine, glycine and proline, negatively correlated with aspartic acid, isoleucine, lysine, glutamic acid, phenylalanine and tyrosine, and uncorrelated with histidine, leucine, methionine, threonine, serine and valine. These correlations, obtained before the bacterial genetic code was elucidated, agree closely with the code, and are similar to those we have found with plant viruses. The correlations we have obtained suggest that our initial assumption, that the gene for the protein of the particles of each virus has a composition similar to that of the whole nucleic acid molecule of the virus, may be largely correct. If true, what does this imply ? It may be that most of the nucleic acid of each virus consists of the gene for the protein in the particle. For satellite virus this is correct, for its nucleic acid is only large enough to code for two proteins of the size of the protein in its particles. However, all the other viruses used had much larger nucleic acid molecules and it might imply that each nucleic acid molecule contained several genes for this one protein. This is very unlikely, as the virus would then probably have several slightly different proteins in its particles, and this seems to be not so. Thus each virus nucleic acid molecule probably contains only one particle protein gene. Therefore Downloaded from www.microbiologyresearch.org by IP: 88.99.165.207 On: Sun, 18 Jun 2017 19:30:55 Amino acid and nucleotide composition 387 our initial assumption, if true, suggests that the different genes of each virus are of similar composition. 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