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Questions: G to A and C to T substitutions, is this a rule ?

Zhongguo Xiong zxiong at arizvm1.ccit.arizona.edu
Mon Apr 11 13:47:09 EST 1994

Dear Colleagues:

Being not familiar with molecular evolution. I have been troubled with some of 
my data in RNA virus sequence. We are working on a small RNA virus (Saguaro 
cactus virus) and nearly complete the sequence of the viral RNA genome from 
cDNA clones. RNA viruses are known to be heterogenous (quasi-species), so it 
was not surprising to see nucleotide sequence variations when sequences are 
obtained from different clones. What was surprising was a consistent rule of 
sequence variaions. G is always subsituted with a A, or vice versa. C is 
always substituted with a T, or vice versa. But there is never a G to (C, T) 
change or vice versa.

Let me try to explain it a little better. We have found 16 nucleotide 
substitutions in about 1500 nucleotide of overlapping sequences. There are 11 
C to T or T to C substitutions and 5 G to A or A to G substitutions. We have 
not found any other possible substituions.

Is there a theory describing the rule  of nucleotide substitution during 
evolution? I feel very ignorant and hope someone can give me a pointer to how 
to explain my observation.

Any comments or suggestions are welcome.

Zxiong at arizvm1.ccit.arizona.edu

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