o make the cellular memory come true, we designed a system with two different strains of E.coli to be our “signal recorder” and “data saver”, two different plasmids to control the process of “write-in” and “erase”, and one plasmid to connect these processes.

hat is memory? What kind of process can be called “remember”? As far as we’re concerned, the memory is to change a transient presence into a long-term existence. Maybe they will have different existing forms, but there will be strict correspondence between them. For example, the persistent expression of a fluorescent protein caused by a transient light stimulus can be regarded as a “memory”.

ut there is also a problem that we can't ignore. As we all know, life can be regarded as a highly efficient machine. There is always a principle to guide its behavior: whether such an approach would make it better to live in this world. If yes, this action will survive, if not, it will be eliminated by the pressure of evolution. In most of the time, the response of the organism to the outside environment is transient. So we can’t to record this brief existence of external factors. Efficient living bodies do not allow themselves to express a certain substance in the condition of no need. This is precisely the key that we need to overcome.

any scientists who are working on this area have their own unique insights on how to solve this problem. But there is a point of agreement whether using the recombinant enzyme or the reverse transcriptase or something else, they all use the DNA, a kind of material which is almost permanent stability in the life body.

o in our project, we also use DNA to storage information, but in an unprecedented way: we use the conjugation to realize the cellular memory.

acterial conjugation is the transfer of genetic material between bacterial cells by direct cell-to-cell contact or by a bridge-like connection between two cells. Discovered in 1946 by Joshua Lederberg and Edward Tatum, conjugation is a mechanism of horizontal gene transfer as the transformation and transduction although these two other mechanisms do not involve cell-to-cell contact.

e use the Escherichia coli K-12(HfrH) strain to be our “Recorder”, if it feels a specific stimulation (the red light in our experiment), it can encode the traI (a necessary protein to the conjugation which has been knocked out in the Bacterial genome), then the oriT (transfer initiation sites, also be knocked out) can lead the reporter gene (GFP in our experiment) transfer from the “Recorder” to the “Saver” through the sex fimbria. And if we want to rewrite the record, we can use a specific artificial signal (the blue light in our experiment), to start the CRISPR_Cas9 system in the “Saver”, which can eliminate the reporter gene efficiently.