WHAT ARE ARCHAEBACTERIA - LIFES 3RD DOMAIN OR MONODERM PROKARYOTES RELATED TO GRAM-POSITIVE BACTERIA - A NEW PROPOSAL FOR THE CLASSIFICATION OF PROKARYOTIC ORGANISMS
Rs. Gupta, WHAT ARE ARCHAEBACTERIA - LIFES 3RD DOMAIN OR MONODERM PROKARYOTES RELATED TO GRAM-POSITIVE BACTERIA - A NEW PROPOSAL FOR THE CLASSIFICATION OF PROKARYOTIC ORGANISMS, Molecular microbiology, 29(3), 1998, pp. 695-707
The evolutionary relationship within prokaryotes is examined based on
signature sequences (defined as conserved inserts or deletions shared
by specific taxa) and phylogenies derived from different proteins. Arc
haebacteria are indicated as being monophyletic by a number of protein
s related to the information transfer processes, In contrast, for seve
ral other highly conserved proteins, common signature sequences are pr
esent in archaebacteria and Gram-positive bacteria, whereas Gram-negat
ive bacteria are indicated as being distinct. For these proteins, arch
aebacteria do not form a phylogenetically distinct dade but show polyp
hyletic branching within Gram-positive bacteria. A closer relationship
of archaebacteria to Gram-positive bacteria in comparison with Gram-n
egative bacteria is generally seen for the majority of the available g
ene/protein sequences. To account for these results and the fact that
both archaebacteria and Gram-positive bacteria are prokaryotes surroun
ded by a single cell membrane, I propose that the primary division wit
hin prokaryotes is between monoderm prokaryotes (surrounded by a singl
e membrane) and diderm prokaryotes (i.e. all true Gram-negative bacter
ia containing both an inner cytoplasmic membrane and an outer membrane
). This proposal is consistent with both cell morphology and signature
sequences in different proteins, The monophyletic nature of archaebac
teria for some genes, and their polyphyletic branching within Gram-pos
itive bacteria as suggested by others, is critically examined, and sev
eral explanations, including derivation of archaebacteria from Gram-po
sitive bacteria in response to antibiotic selection pressure, are prop
osed, Signature sequences in proteins also indicate that the low-G+C G
ram-positive bacteria are phylogenetically distinct from the high-G+C
Gram-positive group and that the diderm prokaryotes (i.e. Gram-negativ
e bacteria) appear to have evolved from the latter group. Protein phyl
ogenies and signature sequences also show that all eukaryotic cells ha
ve received significant gene contributions from both an archaebacteriu
m and a Gram-negative eubacterium. Thus, the hypothesis that archaebac
teria and eukaryotes shared a common ancestor exclusive of eubacteria
is not supported, These observations provide evidence for an alternate
view of the evolutionary relationship among living organisms that is
different from the currently popular three-domain proposal.