Dr. Donald S. Fredrickson 
September 23, 1978 
Page 5 
sine contents of DNA in microorganisms as listed in the Handbook of 
Microbiology (Vol. II, p.585 and following) to provide some guidance. 
Since Appendix A lists bacterial genera on the list of exchangers with 
the exception of a few microorganisms that are also designated by 
species names, I list in Appendix C to this letter the ranges of DNA 
guanine-cytosine contents of the species within these genera as an in- 
dication of the potential relatedness among the microorganisms both 
within and between these various genera. As has been pointed out by 
others, there is DNA sequence homology and evidence of exchange of 
chromosomal genetic information among all members of the genera 
Escherichia , Shigella , Sal monel la , Arizona , Citrobacter , Enterobacter , 
and Klebsiella , although the evidence for chromosomal gene exchange 
and degree of DNA sequence homology between either Klebsiel la or 
Enterobacter and other members of the group are less substantial or 
convincing than evidence for the close relatedness among Escherichia , 
Citrobacter , Salmonella and Shigella . On the other hand, there are 
wide differences in DNA guanine-cytosine contents among the species 
that constitute the genera Serratia and Erwinia (including Pecto - 
bacterium ) and although there is evidence for some Erwinia species of 
genetic exchange with Escherichia coli K-12, there are little or no 
data on exchange for most species in either of these genera. There is 
also inadequate DNA sequence homology and no data on chromosomal genetic 
exchange between either Serratia or Erwinia and members of the 
Enterobacteri aceae other than Escherichia col i . 
The situation is worse in terms of the Pseudomonas species since guanine- 
cytosine contents vary from a low of 48% up to 69% and while there is 
evidence for chromosomal transfer using Inc PI plasmids from only three 
or four species of Pseudomonas to Escherichia coli , no such evidence 
exists for any of the other species of Pseudomonas and certainly none 
with regard to genetic exchange between Pseudomonas species and other 
members of the Enterobacteri aceae such as Salmonella , Shigella , etc. One 
can make additional criticisms based on the information included in 
Appendix C to this letter but the available information really needs to 
be examined by an appropriately constituted panel of impartial experts. 
c. Another problem concerns the issue of whether data are available on 
reciprocity of exchange. For example, although there is evidence for 
transfer and inheritance of chromosomal information from Pseudomonas 
aeruginosa to £. col i K-12, I don't believe that the reciprocal experi- 
ment has been done. In any event, numerous other examples could be given 
since most studies have demonstrated transfer and inheritance to E_. col i 
K-12 and not from E_. col i . I believe it is logical to assume that if 
chromosomal material can be transferred and inherited in one direction 
that it could be inherited in the opposite direction. This assumption of 
reciprocity in the absence of data proving reciprocity should, however, 
be clearly stated. 
[A-312] 
