156
B. M. SHAFFER
ducers of attractor secretion and of adhesiveness, and even perhaps the
trigger inhibitor, are chemically identical. Certainly the first three are
present in crude acrasin (Shaffer, 1956a, b); and indeed this makes the
agar-block test more difficult to carry out. Unless these factors have the
same specificity, it is hard to see how different species can aggregate
separately. However, even the specificity of the attractor has not yet
been definitely accounted for. R. R. Sussman et al. (1958) isolated from
P. violaceum the same three components as they believed to constitute
the attractor complex of D. discoideum, and reported that these would
orient violaceum if they were mixed in drastically different proportions. Yet it is not obvious how specificity of proportions of the same
substances could account for cells being attracted across a foreign
stream (Section IV, A, 1), or for separate cells in the same area being
oriented in opposite directions even though each starts out nearer a
source that does not influence it, unless perhaps secretion were largely
confined to widely spaced pulses. It still seems probable that specificity
lies in the molecules, though theoretically some could be due to differences in sensitivity and the shape of the pulses of secretion (Shaffer,
1957a). An observation on specificity that suggests an interesting line
of attack is that when aggregated cells are dispersed and intermixed, D.
discoideum tends to be competitively inhibited from reaggregating until
P. violaceum cells have removed themselves (Raper and Thorn, 1941).
As aggregation cannot start in sandwich cultures of Acytostelium
without external assistance (Shaffer, 1959; Section IV, B, 1), I have
pointed out that these can be used to test for some component of the
acrasin complex. For although it is possible that streaming may be
induced by many unspecific metabolic stimuli, this is unlikely, because
the rapid movement and the enhanced relays in the sandwich layer are
hardly evidence of general metabolic depression. This second test has
now been used by Hostak (1960), who has found streaming inducible
by some but not others of the steroids active on D. discoideum in my
first acrasin test (Wright and Anderson, 1958), as well as by some
alkaloids. Significantly, higher oxygen tension was ineffective. I have
not found any Dictyostelium or Polysphondylium species in sandwich
cultures to be completely inhibited from triggering aggregation if
illuminated; but P. violaceum is strongly inhibited if kept in darkness,
so it can be employed in this second acrasin test, and perhaps this will
turn out to be true of some other species.
It is by now a biochemical commonplace that the substances on a
particular metabolic pathway themselves control it not only by mass
action but also by positive and negative feedbacks and feedforwards,
influencing enzyme production and also activity. The biochemical significance of aggregation far transcends the mere identification of acrasin
B. M. SHAFFER
ducers of attractor secretion and of adhesiveness, and even perhaps the
trigger inhibitor, are chemically identical. Certainly the first three are
present in crude acrasin (Shaffer, 1956a, b); and indeed this makes the
agar-block test more difficult to carry out. Unless these factors have the
same specificity, it is hard to see how different species can aggregate
separately. However, even the specificity of the attractor has not yet
been definitely accounted for. R. R. Sussman et al. (1958) isolated from
P. violaceum the same three components as they believed to constitute
the attractor complex of D. discoideum, and reported that these would
orient violaceum if they were mixed in drastically different proportions. Yet it is not obvious how specificity of proportions of the same
substances could account for cells being attracted across a foreign
stream (Section IV, A, 1), or for separate cells in the same area being
oriented in opposite directions even though each starts out nearer a
source that does not influence it, unless perhaps secretion were largely
confined to widely spaced pulses. It still seems probable that specificity
lies in the molecules, though theoretically some could be due to differences in sensitivity and the shape of the pulses of secretion (Shaffer,
1957a). An observation on specificity that suggests an interesting line
of attack is that when aggregated cells are dispersed and intermixed, D.
discoideum tends to be competitively inhibited from reaggregating until
P. violaceum cells have removed themselves (Raper and Thorn, 1941).
As aggregation cannot start in sandwich cultures of Acytostelium
without external assistance (Shaffer, 1959; Section IV, B, 1), I have
pointed out that these can be used to test for some component of the
acrasin complex. For although it is possible that streaming may be
induced by many unspecific metabolic stimuli, this is unlikely, because
the rapid movement and the enhanced relays in the sandwich layer are
hardly evidence of general metabolic depression. This second test has
now been used by Hostak (1960), who has found streaming inducible
by some but not others of the steroids active on D. discoideum in my
first acrasin test (Wright and Anderson, 1958), as well as by some
alkaloids. Significantly, higher oxygen tension was ineffective. I have
not found any Dictyostelium or Polysphondylium species in sandwich
cultures to be completely inhibited from triggering aggregation if
illuminated; but P. violaceum is strongly inhibited if kept in darkness,
so it can be employed in this second acrasin test, and perhaps this will
turn out to be true of some other species.
It is by now a biochemical commonplace that the substances on a
particular metabolic pathway themselves control it not only by mass
action but also by positive and negative feedbacks and feedforwards,
influencing enzyme production and also activity. The biochemical significance of aggregation far transcends the mere identification of acrasin