Depending on growth phase and culture conditions, cardiolipin (CL) makes up 5–15% of the phospholipids in
Escherichia coli
with the remainder being primarily phosphatidylethanolamine (PE) and phosphatidylglycerol (PG). In
E. coli,
the
cls
and
ybhO
genes (renamed
clsA
and
clsB
, respectively) each encode a CL synthase (Cls) that catalyzes the condensation of two PG molecules to form CL and glycerol. However, a ∆
clsAB
mutant still makes CL in the stationary phase, indicating the existence of additional Cls. We identified a third Cls encoded by
ymdC
(renamed
clsC
). ClsC has sequence homology with ClsA and ClsB, which all belong to the phospholipase D superfamily. The ∆
clsABC
mutant lacks detectible CL regardless of growth phase or growth conditions. CL can be restored to near wild-type levels in stationary phase in the triple mutant by expressing either
clsA
or
clsB
. Expression of
clsC
alone resulted in a low level of CL in the stationary phase, which increased to near wild-type levels by coexpression of its neighboring gene,
ymdB
. CL synthesis by all Cls is increased with increasing medium osmolarity during logarithmic growth and in stationary phase. However, only ClsA contributes detectible levels of CL at low osmolarity during logarithmic growth. Mutation of the putative catalytic motif of ClsC prevents CL formation. Unlike eukaryotic Cls (that use PG and CDP-diacylglycerol as substrates) or ClsA, the combined YmdB-ClsC used PE as the phosphatidyl donor to PG to form CL, which demonstrates a third and unique mode for CL synthesis.
根据生长阶段和培养条件,磷脂酰肌醇(CL)在大肠杆菌中占磷脂的5-15%,其余主要是磷脂酰乙醇胺(PE)和磷脂酰甘油(PG)。在大肠杆菌中,cls和ybhO基因(分别更名为clsA和clsB)各编码一个CL合成酶(Cls),它们催化两个PG分子的缩合形成CL和甘油。然而,∆clsAB突变体在静止期仍会产生CL,表明存在其他Cls。我们发现由ymdC编码的第三个Cls(更名为clsC)。ClsC与ClsA和ClsB具有序列同源性,它们都属于磷脂酶D超家族。无论在生长阶段还是在生长条件下,∆clsABC突变体都缺乏可检测到的CL。通过表达clsA或clsB,可以在三重突变体的静止期中将CL恢复到接近野生型水平。仅表达clsC会导致静止期中CL的水平较低,通过共表达其相邻基因ymdB,CL的水平会增加到接近野生型水平。在对数生长期和静止期中,所有Cls合成的CL随着培养基渗透压的增加而增加。然而,在对数生长期低渗透压下,只有ClsA会贡献可检测到的CL。ClsC的假定催化基序突变阻止了CL的形成。与使用PG和CDP-二酰基甘油作为基质的真核Cls或ClsA不同,结合的YmdB-ClsC使用PE作为磷脂基质将PG转化为CL,这证明了CL合成的第三种独特模式。