The murein wall in Gram-negative bacteria is so thin that the mechanis
m of growth is necessarily complicated. From analytical data of murein
components, Holtje suggested a model for the growth mechanism that wo
uld lead to safe wall enlargement. The model depended on the formation
of trimers of peptidoglycan disaccharides linked via their pentapepti
des. In the 'three-for-one' model three oligopeptidoglycan chains are
linked to each other in the usual linkages between the carboxyl group
of D-alanine residues and the E-amino group of diaminopimelic acid res
idues; these are designated 'tail-to-tail' linkages. This three-chaine
d raft is then linked to the stress-bearing wall via the formation of
trimers, defined as three peptide chains linked together by tail-to-ta
il linkages. Then by autolyzing the oldest bonds in each trimer, the o
ld chain is excised and the raft becomes part of the stress-bearing wa
ll and the wall is enlarged. There is a problem with the three-for-one
model in that it demands a precise fitting of the prefabricated raft
of three crosslinked chains to a stress-bearing chain in the wall fabr
ic to allow the series of trimer linkages to form. Because the wall, w
hen bearing stress, must be pulled into a 'honeycomb' structure, the e
nd-to-end distance would be shortened. The possibility is raised here
that the glycan chains in the stress-bearing wall are stretched to a s
ufficient degree by the cell's turgor pressure to compensate for its z
ig-zag structure; this could allow the model to function. A calculatio
n is presented that assumes that the area of the pores in the fabric,
called tessera, is maximized by the cell's turgor pressure. In this ca
se the glycan chain must stretch 10% (and the end-to-end distance of p
eptide strands stretch 28%) so that the end-to-end distance of a glyca
n chain in the stress-bearing wall and the unstretched nascent wall ca
n be the same and permit indefinite stable growth. (C) 1998 Federation
of European Microbiological Societies. Published by Elsevier Science
B.V.