Sending DNA Messages Inside Viruses

Every living thing on the planet is either a single-celled organism or a multicellular organism made of many cells. The power of cellular life is that cells, each tiny, communicate with one another to act collectively. Cells in the animal body share a single circulatory system that is used to convey hormonal chemical messages, and neurons transmit electrical messages from one cell to the next. Even bacterial cells, far from being independent agents, communicate with a wide array of chemical signals, to coordinate population- and community-level behaviors.

Given its power, it is no surprise that synthetic biologists want to harness the power of intercellular communication. I've even used it in my own work, in the form of quorum sensing. Quorum sensing is the basic model of bacterial intercellular communication. Cells continuously produce a small molecule at a constant rate. When isolated, the molecules are at a low concentration. When many cells, each producing the molecule, are packed together, the concentration grows and can regulate the expression of genes that require group behavior.

A recent paper¹ opens up the possibility of designer intercellular messages that are more sophisticated than the on/off quorum sensing switch. Instead of using small molecule messages, they send entire genes packaged inside the M13 phage, a virus that infects E. coli. Viruses are great at shuttling messages from one cell to another: it's how they make their living. All viruses carry the message "copy me," allowing them to put their host cell to work churning out more viruses. To the authors' benefit, M13 can package foreign DNA sequences inside itself, as long as they contain a unique marker sequence. Because of this property, the pieces of DNA are called phagemids (a play on "phage" and "plasmids.") Another benefit of the M13 system is that only cells carrying F-plasmids can be infected, and only cells carrying a helper plasmid (M13K07, or just M) can send messages. F-plasmids (F for "fertility") are spread through conjugation, where bacteria exchange DNA from cell to cell through a tube called a sex pilus. So, to sum up: genetic messages marked with a special sequence can be packed inside M13. These messages can only be sent from bacteria carrying helper M-plasmids (by "willing" senders) and taken up by bacteria carrying F-plasmids (by "willing" recipients). Thus, there is a great deal of control over who can send or receive, and the content of the DNA messages.

This paper is a proof-of-concept, showing that M13 can be used to transmit genetic messages between cells. The authors tested their system by sending several different messages. In their first experiment, a culture was started with a mixed population of sending cells and receiving cells. The sending cells contained GFP with the "send" marker, and the receiving cells produced RFP (GFP = green fluorescent protein; RFP = red fluorescent protein). Five hours after starting the experiment, the large majority of cells were producing either RFP and GFP or GFP only (see the lower figure). This means that (1) most of the receiving cells got the message to produce GFP in addition to the RFP they were already producing, and (2) the sender cells did not erroneously get the message to produce RFP back from the receiver cells (which did not contain M-plasmids, and RFP was not marked for M13 packaging). The authors did find however that occasionally the M-plasmid will be sent as a message, turning receiver cells into "rebroadcasting" cells. I'm sure some interesting applications of that fact exist.

In a second experiment (see the top figure), M13 carried the message to produce T7 RNA polymerase (T7 RNAP). T7 RNAP is a viral RNA polymerase, so it only reads the T7 promoter, not the native E. coli promoter. Likewise, E. coli's own RNA polymerase does not read genes regulated by the T7 promoter. The T7 RNAP gene is sent via M13 to receiver cells with a reporter gene regulated by the T7 promoter. Infection by M13 "unlocks" the reporter by causing the cell to express T7 RNAP, and thus producing the reporter protein.

Is there anything useful in this, or is it merely an academic exercise? For starters, communication is incredibly important in the biological world. As we continue to gain competency in engineering it, we will want hold of the same tricks it uses. However, living things do not use genetic material as a means of communication; this is an engineered tool. That means it is completely insulated from endogenous cellular communication systems, and therefore more "engineerable." Perhaps this technology could even be used in a radical re-thinking of regulating gene expression. Rather than regulating when genes are turned on, we could perhaps control when cells get a gene in the first place.

Image credits: from ref. 1

Reference:

1. Ortiz, M. E. & Endy, D. Engineered Cell-Cell Communication via DNA Messaging. Journal of Biological Engineering 6, 16 (2012).