  The ability to learn and respond to recurrent events depends on the capacity
to remember transient biological signals received in the past. Moreover, it may
be desirable to remember or ignore these transient signals conditioned upon
other signals that are active at specific points in time or in unique
environments. Here, we propose a simple genetic circuit in bacteria that is
capable of conditionally memorizing a signal in the form of a transcription
factor concentration. The circuit behaves similarly to a "data latch" in an
electronic circuit, i.e. it reads and stores an input signal only when
conditioned to do so by a "read command". Our circuit is of the same size as
the well-known genetic toggle switch (an unconditional latch) which consists of
two mutually repressing genes, but is complemented with a "regulatory front
end" involving protein heterodimerization as a simple way to implement
conditional control. Deterministic and stochastic analysis of the circuit
dynamics indicate that an experimental implementation is feasible based on
well-characterized genes and proteins. It is not known, to which extent
molecular networks are able to conditionally store information in natural
contexts for bacteria. However, our results suggest that such sequential logic
elements may be readily implemented by cells through the combination of
existing protein-protein interactions and simple transcriptional regulation.
