Tom Dougherty Doughert at
Mon Feb 5 18:07:01 EST 1996

In article <4f31u4$5do at>, kisand at (Veljo Kisand) says:
>Tom Dougherty (Doughert at wrote:
>: In article <4ei11i$ioo at>, kisand at (Veljo Kisand) says:
>: >
>: >an456680 at wrote:
>: >>: Hi all,
>: >: I'm trying to grow some plasmids in E.coli and hopefully radio label
>: >: the DNA. I'm told I need some thymidine (thymine??) free media, so
>: >: that I can add some radio labelled thymidine to it.

  An outstanding discussion of the problems and pitfalls of
 thymine/thymidine metabolism is in Bacteriol. Revs. 34:278-343.  It is still the
 best discussion of this topic.  The amount of radiolabeled thymidine incorporated
 is very sensitive to the internal pool level of deoxyribose sugar, and to whether
 the E. coli strain is a thymidine requiring mutant. 
>Well, I didn't want to say that internal pools of thymidine/thymine don't
>influence use of external pools. And I'm not specialist of E.colis but at least
>i know most of bacteria prefer salvage pathway to de novo synthesis... 
> If specific activity of yourisotope is high enough your label
 some of DNA  It doesn't really matter if thymidine is degradated or not,
> labelling site is bound to aromatic ring.
>Worser is situation when thymidine is used for energetic purposes, then your
>nice label happens to be who knows where. That means "real" degradation.
>Olny thing with this internal pools is that you should grow bacteria long
>enough on label so some isotope can reach to DNA...
>Veljo Kisand

Again, I will say, please read the review.  E. coli normally makes dTMP from
dUMP. This is subsequently phosphorylated to dTDP and finally dTTP.
Thymidine is NOT in the normal biosynthetic pathway.  It is present only when
added exogenously.  Check out the diagram on page 464 in "Escherichia
coli and Salmonella typhimurium, cellular and molecular biology" ASM Press.
The discussion in the same book on page 466 sums up utilization of 
thymine and thymidine.   The label on the thymidine is on the base, and not the
sugar.  This means that radomization of the label does not occur, as the 
sugar moiety is the part that is broken down and metabolised extensively.     

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