Q.80 Among the following characteristics listed below, select those that are the most appropriate
for archaebacteria.
(i) Peptidoglycan containing muramic acid and D–amino acids are present in cell
wall
(ii) Long chain branched alcohols (phytanols) bound to glycerol by ether linkages
are found in the lipids of cytoplasmic membrane
(iii) First amino acid to initiate new polypeptide chain is N–Formylmethionine
(iv) Translation process is sensitive to the action of diphtheria toxin
(A) (i), (iii) (B) (ii), (iii), (iv) (C) (ii), (iv) (D) (i), (ii), (iv)
The correct answer is (B) (ii), (iii), (iv).
Archaebacteria, or Archaea, possess unique features distinguishing them from eubacteria, particularly in cell wall, membrane lipids, and protein synthesis mechanisms. These traits are key for competitive exams like CSIR NET Life Sciences.
Option Analysis
(i) Peptidoglycan with muramic acid and D-amino acids
Archaea cell walls lack true peptidoglycan, which defines eubacterial walls and contains N-acetylmuramic acid (NAM) linked β-1,4 to N-acetylglucosamine, with D-amino acids like D-Ala and D-Glu for cross-linking. Archaea use pseudomurein (in methanogens, with NAT instead of NAM, β-1,3 links, L-amino acids), S-layers, or glycoproteins, making this characteristic incorrect for archaebacteria.
(ii) Ether-linked phytanols in membrane lipids
Archaea cytoplasmic membranes feature isoprenoid chains like phytanols (C20 branched hydrocarbons) bound via ether linkages to sn-glycerol-1-phosphate (G1P), enhancing stability in extreme environments unlike bacterial ester-linked fatty acids. This is a hallmark archaeal trait.
(iii) N-Formylmethionine initiates polypeptide chains
Unlike eubacteria using N-formylmethionine (fMet), archaea initiate translation with unformylated methionine, similar to eukaryotes, as their initiation resembles eukaryotic systems without bacterial formyltransferase.
(iv) Translation sensitive to diphtheria toxin
Archaea EF-2 contains diphthamide, a modified histidine making it susceptible to diphtheria toxin ADP-ribosylation, inhibiting translation like in eukaryotes, unlike resistant bacterial EF-G.
Archaebacteria characteristics set them apart in the tree of life, especially regarding peptidoglycan muramic acid absence, phytanols in ether linkages, N-formylmethionine in translation, and diphtheria toxin sensitivity. These features are crucial for CSIR NET Life Sciences aspirants studying microbial diversity.
Cell Wall: No Peptidoglycan or Muramic Acid
Standard textbooks note archaebacteria cell walls lack peptidoglycan with muramic acid and D-amino acids—exclusive to eubacteria. Instead, pseudomurein or proteins provide rigidity.
Membrane Lipids: Ether-Linked Phytanols
Long-chain branched phytanols link via ether bonds to glycerol, enabling extremophile survival, contrasting bacterial ester lipids.
Translation Initiation: Not N-Formylmethionine
Archaea use methionine, not N-formylmethionine like bacteria; their system aligns with eukaryotes.
Diphtheria Toxin Sensitivity
Archaea translation halts under diphtheria toxin due to EF-2 diphthamide, unlike bacteria.
