and c-signaling requires cell movement for its transmission from one cell to another
(Kroos et al., 1988)(Kim, 1990a). C-signal deficient mutants (csgA) were found to
grow and swarm normally but they failed to aggregate or to sporulate after starvation.
CsgA protein is 25 kD and is secreted to the cell surface where it is cleaved to the
active 17 kDa C-signal (Kim, 1990b) by a membrane protease (Lobedanz & Søgaard-
Andersen, 2003) (Rolbetski et al., 2008). C-signal transfer is brought about by
forceful collisions between pairs of aligned cells actively moving into end-to-end
(Kim, 1990c) (Nudleman et al.,2005) (Wall et al.,1998a)(Wall et al.,1998b)
(Wei et al., 2011).
When development starts there are only a few C-signal molecules per cell.
However, expression of the C-signal increases rapidly due to a positive feedback loop
involving the five proteins of the act operon (Gronewold & Kaiser, 2001). Whenever a
cell reverses in response to its pacemaker, it would come into end-to-end contact with the
cell immediately behind it that is still moving toward a nascent fruiting body. Such
forceful contact should allow the 2 cells to exchange the C-signal. Each time C-signal is
exchanged between cells in an aggregation stream, the positive feedback loop would
increase expression of csgA and elevate the number of signal molecules on both signaling
cells (Gronewold & Kaiser, 2001). Spore differention is likely to be the final step in
fruiting body development because spores have lost their poles and without their polar
engines would no longer be able to propel themselves and to raise the level of C-signal
further. Sporulation is triggered only after cells had been signaling each other long
enough for the level of the C-signal to have reached some elevated threshold, ensuring
that only the most fit cells become spores inside the nascent fruiting body. The spatial
distribution of spores found within a mature fruiting body fits having C-signal rise to
some threshold level before sporulating (Harvey et al., 2013).
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