In article <1994Apr24.131637.23432 at dal1>, arlin at ac.dal.ca wrote:
> In article <timi-210494183304 at kustu1.berkeley.edu>, timi at mendel.berkeley.edu (Tim Ikeda) writes:
> > In article <1994Apr21.190914.23337 at dal1>, arlin at ac.dal.ca wrote:
> >> If these and a few other important characters (circular chromosomes,
> >> operons, shine-dalgarno sites, etc) were present in the common
> >> ancestor, as is likely, then archaebacteria and eubacteria have a
> >> common heritage that I would call "bacterial." [...]
> > Just an aside...
> > It's time to throw out the "bacteria only have circular chromosomes" idea.
> > See: Davidson BE; MacDougall J; Saint Girons I. 1992.
> > The cool question is: What system/s does a bacterium use to maintain a
> > linear chromosome?
>> Certainly its a cool question, but saying (as I did) that bacteria
> *ancestrally* have circular chromosomes is not the same as saying
> "bacteria only have circular chromosomes" (Tim's words, not mine).
> Linear bacterial chromosomes are, as far as I know, restricted to
> members of the spirochete genus _Borrelia_, and the inference that
> circular chromosomes are ancestral is therefore a sound one.
While i agree that circular chromosomes are likely to be ancestral to the
bacterial lineage, linear chromosomes and linear plasmids are widespread
among bacterial taxa. Aside from Borrelia (Casjens Mol Micro, May 93), i
can recall that Agrobacterium (J Bact, Dec 93) and Streptomyces (MGG, Nov
93) also have linear chromosomes. I also recal sherwood casjens popping up
a slide with numerous linear-chromosome bearing lineages, widespread among
eubacteria. in each case, however, sister-taxa bore circular chromosomes,
leading to the conclusion the linearity was a derived character.
dept of biology
university of utah
lawrence at bioscience.utah.edu