The next short section is titled Current model of DNA uptake. It presents a new version of the figure I posted yesterday, with the gene names and the nucleotide import removed. It describes what we think happens in gram-negative bacteria, relying on evidence mainly from H. influenzae and the Neisserias. The basic steps are described, but the detailed evidence and the problems aren't pointed out until the Gaps sections.
- Initiation occurs internally on dsDNA fragments, preferentially at short sequence motifs called uptake sequences.
- The force for DNA uptake is produced by retraction of cylindrical protein multimers called pseudopili, short versions of long extracellular filaments called type IV pili.
- Transforming DNA enters the periplasm through outer membrane 'secretin' pores like those through whch type IV pili exit.
- DNA may bind nonspecifically to the pseudopilus, as it does to type IV pili, and this may be how the force of pseudopilus retraction is transmitted to the DNA.
- Once DNA is in the periplasm, a fragment end interacts with the translocation machinery. One DNA strand is degraded to nucleotides as the other enters the cytoplasm.
This model is superficially satisfying, but a critical analysis reveals a number of problems. Next I describe the three serious gaps that will be filed by the experiments I propose.
Gap 1. What are the players? (What proteins contribute to DNA uptake? Which of these interact directly with DNA?)
The first paragraph explains that H. influenzae is particularly appropriate for this analysis because (i) we've identified all the members of its competence regulon (can't be done in Neisseria (no regulation), or the Gram-positives (no competence-specific regulation).
The next two paragraphs describe what's known and not known about the H. influenzae competence genes; it includes a little figure showing all the genes clustered under their likely roles. Only four genes can reasonably be excluded from roles in uptake. Four others have no known function, and most of the rest have only suggested functions. Only six are directly implicated in uptake by assays of labeled DNA and phenotypes of non-polar knockout mutations.
Another paragraph describes the need to identify the proteins that bind DNA, and the limited success to date (some evidence of non-specific binding).
This section ends with a paragraph describing the Specific Aim and the first two questions: which proteins contribute to DNA uptake (and translocation), and which proteins contact DNA during uptake and translocation. I think the paragraph should also summarize the strategy.
Gap 2. What is the uptake specificity? How does it act?
The first paragraph will give basic factoids about uptake specificity and uptake sequences. Both H. influenzae and N. gonorrheoeae have long been known to have very strong preferences for DNA from close relatives. We now know that this is due to an uptake bias favouring sequence motifs abundant in these DNAs; our measures show at least a 100-fold bias (published reports are inconsistent). The preferred H. influenzae sequence was initially identified as a 9 bp AAGTGCGGT (N. gonorrhoeae's is XXXXXXXXX). However once genome sequences became available the focus shifted to characterizing the many copies of these sequences in the respective genomes. Once my lab realized that the genome sequences are not replicative elements but motifs (like other protein-binding sites), we shifted to characterizing them as such (see attached manuscript).
Relevance of uptake sequences to the mechanism of uptake: 1. Uptake sequences are likely to result from general features of DNA uptake in Gram-negative bacteria. Even though their sequences are different the genomic H. influenzae and Neisseria uptake sequences share almost all other properties: frequency, spacing, frequent accessory role as transcription terminators, strong consensus (see attached manuscripts). These are the two best studied DNA Gram-negative uptake systems. Their uptake specificities are also shared by other members of the Neisseria, and by all Pasteurellaceae. There is also some evidence of uptake specificity in other systems (Campylobacter, others?), and most have not been examined. The absence of obvious uptake-sequence-like repeats in genomes doesn't mean that the species' DNA uptake machinery has no sequence specificity. The failure to easily find a single protein that binds specifically to uptake sequences (in either H. influenzae or N. meningtidis) suggests that uptake specificity is not a detachable 'add-on' to the mechanism (a kind of pre-screening) but is rather intrinsic to the process.
One big problem with the uptake model presented above is the need for sharp DNA kinking, and that may be resolved by uptake sequences. Double-stranded DNA is quite stiff on the scale needed for uptake, with a persistence length of >50 nm (for comparison, the secretin pore has a diameter of about 6-7 nm ). (Can this be indicated on the figure; is the figure drawn roughly to scale?) The model shows a slight bend at the uptake sequence (structural models predict a ??° bend at each of the two AT-rich segments in the H. influenzae uptake sequence, and we see the predicted gel retardation in a 200 bp model fragment), but clearly a sharp kink of nearly 180° is needed for passage through the pore. We know that uptake does not need to occur at the ends of fragments, because closed circular plasmids are taken up as efficiently as linear fragments. Current understanding of DNA structure is not good enough to predict whether uptake sequences are preferential kink sites , nor what kind of force might be needed to cause it. Because of DNA's high charge, bypassing the pore is not an option. (There's also the problem of fitting the DNA in the pore beside the pseudopilus, but it may be better to not even bring this up as I don't propose to solve it.)
Say that we can't just rely on the genomic uptake sequence motif as a surrogate for the uptake specificity. Evidence in the manuscript that uptake bias only imperfectly corresponds to the genomic motif, also evidence of strain-to-strain variation at uptake sequence positions doesn't correspond.
I propose to carry out a very high resolution analysis of uptake specificity, examining both sequences in the uptake sequence motif and effects of nearby sequences. The high resolution will allow investigation of effects on uptake by interactions between different positions in the motif (like the genomic covariation analysis in the manuscript). We will also investigate the physical properties of the newly defined uptake motif. Finally, we will exploit the variant specificity of the related A. pleuropneumoniae DNA uptake system in a cross-species complementation experiment to identify the genes responsible for their different uptake specificities.
Gap 3. What forces does DNA experience during uptake?
Force must act on DNA to create the kink needed to initiate uptake; force is also needed to continue uptake. The model shown above addresses this by using pseudopilus retraction to pull DNA across the outer membrane, but it overlooks two big problems, the need for a ratchet and the absence of the retraction protein PilT.
In principle, retraction using a type IV pilus mechanism is certainly able to generate a sufficiently strong force; measurements using optical tweezers (where a cell is stuck to a coverslip and its pilus is attached to a bead in the optical laser trap) have recorded forces in excess of 150 pN, the strongest molecular forces known. The details of pilus assembly and retraction are shown in the figure (??? maybe we'll have a figure here). Prepilin subunits in the inner membrane are freed from their leader sequence by a prepilin peptidase (pilD), and assembled into the base of the elongating pilus by the PilB ATPase. Retraction occurs by disassembly of the subunits by the reverse of this reaction, typically catalyzed by the related PilT ATPase; this is where the force is generated. The entire complex is thought to be restrained in the inner membrane by other proteins (name them??).
Although H. influenzae has good homologs of all the other Tfp proteins, it lacks any identifiable homolog of PilT, as do all of its Pasteurellacean relatives. Thus we do not know where the force comes from. (One strain has been shown to be capable of assembling and retracting pili under special conditions, so we know it has all the required genes; to date thee recognized ones are all int the competence regulon.) We will take two approaches to finding the source of the force. First, characterization of all proteins that interact with incoming DNA should identify it, even if it's not one of the known competence proteins. Second, using optical tweezers to characterize the forces acting on DNA during uptake will show whether the force has the properties expected of a Tfp mechanism.
The second problem with the current model of uptake is the need for a ratchet. This problem has been largely overlooked by the Neisseria researchers, perhaps because their main focus has been on the role of Nesseria's long type IV pili in pathogenesis. However, type IV pili have not been detected on H. influenzae cells under normal growth conditions, and even though competent cells dramatically upregulate all of the Tfp genes they do not have detectable pili. Neisseria cell also do not need long pili to take up DNA, as mutants defective in pilus assembly are proficient for uptake. Many other naturally competent Gram-negative bacteria also lack detectable type IV pili, despite possessing the same genes as H. influenzae (though they do have PilT).
Type IV pili can be several µ long, so a single pilus retraction could in principle pull in DNA fragments as long as 10 kb (if one end of the DNA bound to the proxinal end of the pilus) (see 'Not this' figure). But DNA is normally taken up not by pili but by pseudopili, which are thought to only span the distance between the inner and outer membranes (??? 20 nm???), and not to protrude significantly beyond the cell surface. Thus a single pseudopilus retraction can only be expected to pull in about 100 bp of DNA (see 'But this' figure).
One possible solution would be coupling of translocation to uptake, with the pseudopilus only needed to bring some part of the DNA into contact with the translocation machinery. But we know this is not the solution in H. influenzae, because circular plasmids can be fully taken into the periplasm even though they cannot be translocated, and because uptake proceeds normally in a rec2 mutant, which cannot translocate DNA.
The simplest solution (thought by no means simple) is for the pseudopilus to act as a ratchet, alternately elongating and binding DNA and then retracting and releasing the DNA. A detailed drawing of this mechanism is provided in the Appendix. The timescale of DNA uptake (a few minutes?) and the length of each retraction event makes this potentially detectable with optical tweezers.
In addition to their use for characterizing pilus retraction, optical tweezers have been used to measure forces on DNA during uptake by competent cells. This work shows that this is a good way to measure forces but was not informative about ratchet mechanisms because the bacteria used don't face this problem. The first measurements were done with B. subtilis, which needs its pseudopilus only to bring a part of the DNA to the cell surface where it is cut for translocation (mutants lacking the pseudopilus proteins and translocate DNA provided the cell wall barrier is removed). The only other tweezer measurements have been done with Campylobacter, which is (with its relative Helicobacter) the only uptake system that doesn't use Tfp machinery.
(Say more about forces here). (Also say that maybe the need for sequence specificity arises from the complications introduced by the outer membrane and the need for an uptake ratchet??)
Now describe what I propose to do. Need details here.
Other ideas to include somewhere else:
If we detect binding of a particular protein to DNA, we can then test whether mutations in other proteins affect this binding - this might help define the order of events within, e.g. DNA uptake. For example, if we see that pilin does contact DNA, we can test whether a secretin mutation prevents this.
If sequencing costs drop we will repeat the Q. 3 analysis of uptake specificity with A. pleuropneumoniae cells and their USS.