Incorporation of the disaccharide pentapeptide into the existing
sacculus takes place dispersively, i.e., over the whole
surface of the lateral wall (for reviews, see references 80, 122,
and 132). In other words, cell growth is not polarized as, for
instance, in Schizosaccharomyces pombe (106). Impairment of
PBP2, either by mecillinam or by growing a temperature-sensitive
pbpA (the gene coding for PBP2) mutant at the nonpermissive
temperature, leads to spherical cells; the same shape
was observed in a rodA mutant. In temperature-sensitive rodA
mutants, PBP2 activity is reduced when measured in a cell-free
system (73). These authors have therefore suggested that PBP2
and RodA interact to maintain the rod shape. The enzymatic
function of RodA is not known. However, it contains transmembrane
sequences similar to the cell division protein FtsW
(see below) and to SpoVE of B. subtilis (for a review, see
reference 102).
Specific inhibition of PBP2 by the antibiotic mecillinam
strongly reduces meso-[3
H]diaminopimelic acid ([3
H]DAP) incorporation
into peptidoglycan. Inhibition of PBP1a and
PBP1b by cefsoludin is much less severe, which has led to the
idea that PBP1a and PBP1b provide primers for PBP2 to act
upon (177). E. coli can also grow in the absence of a functional
PBP2 when the intracellular level of ppGpp (guanosine tetraphosphate,
the alarmone of the stringent response) is elevated
(75). In this case, the cells are round, suggesting that the role
of PBP2 has been taken over by PBP3. It is believed that
spherical cells contain polar cap material (69).