Curli Extracellular Matrix - The Functional Fold: Amyloid Structures in Nature - page 118

from wild-type fibres, but less abundant. It is speculated that csgF

mutants are defective in CsgB functioning, while csgE encodes for a

4,12

chaperon of CsgA.

Environmental
signals

Environmental
signals

G G G F F F E E E D D B B B A A A C C

accessory
genes

accessory
genes

Regulating
gene

Regulating
gene

Structural
genes

Structural
genes

Figure 7.1 Schematic view of genes cluster. Correct secretion

and assembly of curli extracellular matrix depends on the
expression of both accessory and structural genes. A variety
of environmental signals affect the transcription of curli
accessory genes from the

csg

operon. Once induced, CsgD
positively regulates the transcription of the structural genes
from the second operon,

csgDEFG

csgBAC .

7.3 From Regulation to Secretion

7.3.1 Regulation of Curli Expression

Several environmental conditions were reported to promote curli
expression: temperature below 30°C

13

(though it was shown that

significant fraction of E . coli isolates from human blood cultures

14

did produce curli at 37°C); depletion in nutrients as phosphate,

15,16 5,15 15

nitrogen, and iron; oxygen tension; presence of ethanol (4%);

17,18

and low osmolarity. In contrast, excess of N-acetylglucosamine-

6-phosphate (GlcNAc-6P) was reported to downregulate curli
expression.

19

Several cellular factors were reported to influence
curli expression: RpoS (stationary phase sigma factor) — induces
curli expression during stationary phase; Crl — a transcription factor
that is required for curli expression;

13

MlrA (MerR-like regulator)

— a transcriptional regulator of csgD that positively regulates curli

20

expression; OmpR (outer membrane protein R) — together with

EnvZ protein — they form a two component system that positively
regulates

16,21,22

CpxA/
R two component system and Rcs — these systems negatively
regulate curli expression in response to envelope stress;

csgD in response to changes in osmolarity;

IHF (integration host factor) this protein was shown to act as

17,23-27