Humans draw the LINE at Gonorrhea. Not that it helps.
א וַיְדַבֵּר יְהוָה, אֶל-מֹשֶׁה וְאֶל-אַהֲרֹן לֵאמֹר. ב דַּבְּרוּ אֶל-בְּנֵי יִשְׂרָאֵל, וַאֲמַרְתֶּם אֲלֵהֶם: אִישׁ אִישׁ, כִּי יִהְיֶה זָב מִבְּשָׂרוֹ–זוֹבוֹ, טָמֵא הוּא. ג וְזֹאת תִּהְיֶה טֻמְאָתוֹ, בְּזוֹבוֹ: רָר בְּשָׂרוֹ אֶת-זוֹבוֹ, אוֹ-הֶחְתִּים בְּשָׂרוֹ מִזּוֹבוֹ–טֻמְאָתוֹ, הִוא. ד כָּל-הַמִּשְׁכָּב, אֲשֶׁר יִשְׁכַּב עָלָיו הַזָּב–יִטְמָא; וְכָל-הַכְּלִי אֲשֶׁר-יֵשֵׁב עָלָיו, יִטְמָא. ,
The day after Valentine’s Day. Ah! What better day in the year can we find to discuss gonorrhea? In the US alone 700,000 people are infected each year, and 5 million are infected worldwide. In most infected men gonorrhea causes urethral discharge and pain while urinating. The reason is that Neisseria gonhorrea have little hair-like structures called fimbriae. This makes them very sticky and they stick to the urethra’s walls. Then you get inflammation, urethritis and urination becomes difficult and painful. In women, if left untreated, gonorrhea can lead to pelvic inflammatory disease, a painful condition that can cause sterility or ectopic pregnancy.
For those of you who are wondering, gonorrhea actually predates college dormitories by a few millenia. The opening paragraph of this post is taken from Leviticus 15, 1-4. The first 15 verses of this chapter deal with male discharge, presumably gonorrhea. Strict quarantine and washing procedures were required from infected men. That’s a lot of verses and instructions. Gonorrhea must have been considered to serious problem back then, as now. The Ebers papyrus, dating back to ~1500 BCE also mentions cures for burning urination.
I certainly hope you used condoms last night.
Recently, researchers at Northwestern Medical School have found evidence of a human DNA fragment in Neisseria gonorrhoeae. The gene transfer appears to be quite recent, in evolutionary terms. So “they” have a bit of “us” in them. Given that N. gonorrhoeae have been sticking to mankind for thousands of years, human DNA in the bacterial genome is more than plausible: it is almost expected. The human DNA was found in three of fourteen bacterial isolates that were sequenced. The question is: does the human DNA provide fitness to the bacteria? Probably not. But it may just provide fitness to itself, as selfish DNA: DNA which spreads by forming additional copies of itself, and has no positive contribution to the reproductive success of the organism. The human DNA found in N. gonorrhoeae fits the description of selfish DNA. This bit of DNA, nicknamed LINE is found in 500,000 copies in the human genome. Long Interspersed Nuclear Elements or LINEs are transcribed from DNA to RNA by an RNA polymerase II enzyme which they encode. LINEs code for a reverse transcriptase: the enzyme that can get the RNA encoded as DNA in another place in the genome. Since LINEs code for their own genome integration, they are actually a semi-autonomous replicating element within the human genome. A small “genome” within a genome, constantly replicating and embedding itself. Indeed, over the years we have amassed some 500,000 LINEs in our genome, making up 17% of our total genomic content. Think of LINEs as the Tribbles of evolution, replicating themselves constantly. It seems that now LINEs have jumped species, and are infecting our infectors. (UPDATE: Thanks to Joe H for the correction. LINEs do not encode for RNA Polymerase II, they use the human one.)
The similarity between the LINEs in the bacteria and the human LINEs is very high. The authors hypothesize that 1) the gene transfer may have happened recently or 2) LINEs are extremely conserved in the bacteria due to some selective pressure. A third hypothesis is that this gene transfer happens multiple times, with a high frequency. In any infected host, there is a certian probability that LINE (or another human genomic element) shall be transferred from human to Nisseria, then replicate with the bacteria for a while (maybe thousands of generations), but eventually drop out of the bacterial genome. Bacteria don’t tolerate too much dead weight in their genome, unlike eukaryotes. But just as humans get reinfected with gonorrhea all the time, we keep transferring our genomic elements to them.
I think it is high time we looked for LINEs in other human pathogens, and for more genomic evidence of host-pathogen DNA transmission. Finally, at this point, I would like to let Frank Zappa continue our discussion (slightly NSFW.)
Mark T. Anderson, & H. Steven Seifert (2011). Opportunity and Means: Horizontal Gene Transfer from the Human Host to a Bacterial Pathogen mBio, 1-4 : 10.1128/mBio.00005-11