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\b[Artificial recruitment separates recognition of an <RNA> target from the activity of the recruited regulator.] In an <RNA tethering assay>, a reporter carries binding hairpins for a defined RNA-binding module, such as <MS2 coat protein>. Fusing the candidate regulator to that module places it on the reporter without requiring its normal sequence-recognition mechanism. The experiment asks whether recruitment is sufficient to change <RNA> processing, localization, stability or <biological translation>, and which parts of the regulator produce that change. It does not establish that the endogenous <protein> normally finds that <RNA> or that artificial occupancy has a physiological affinity.
A basic design compares a reporter with and without intact hairpins, expressing either the coat <protein> alone or the fusion at matched levels. Measure reporter <RNA> and <protein> separately, with an untethered reporter as an internal control. <protein> output alone is ambiguous: a reduction can reflect fewer <RNA> molecules, lower <biological translation> per molecule, altered <protein> stability or general toxicity. In a simple constant-rate model,
$$
\frac{dM}{dt}=s-k_dM,\qquad \frac{dP}{dt}=k_tM-k_pP,\qquad P^*=\frac{s k_t}{k_d k_p},
$$
where $M$ and $P$ are <RNA> and <protein> amounts, $s$ is <RNA> synthesis, and the other coefficients describe <RNA decay>, <biological translation> and <protein> loss. Thus one <protein> endpoint cannot identify which rate changed. Time-resolved <RNA> measurements, poly(A)-tail analysis and a direct synthesis assay discriminate these possibilities. Place hairpins in several non-disruptive positions and vary their number, because an array can change <RNA> structure, sterically impede machinery or create nonphysiological <multivalency>. Confirm fusion integrity, localization and <RNA> association; a nonfunctional fusion is not evidence that the candidate lacks regulatory activity.
A foundational application concerned <poly(A)-binding protein>. <Jeffery M. Coller> and colleagues tethered yeast Pab1p to a reporter's 3′ region and showed stabilization even without a <poly(A) tail>. Domain analysis separated the stabilization activity from the domains required for normal poly(A) recognition, while <biological translation> dependence showed that protection was coupled to the translated state.<mRNA stabilization by poly(A) binding protein is independent of poly(A) and requires translation by Jeffery M. Coller and others>[Tethered Pab1p, tail-independent stabilization and domain tests]{ref} The important inference is that an <RNA> element can act by recruiting an effector: the poly(A) sequence is not intrinsically required for every downstream stabilizing action. This is stronger mechanistic information than a correlation between long tails and stable <RNA>, although artificial recruitment does not imply that natural <poly(A) tails> have no other functions.
Tethering also revealed positional logic in <nonsense-mediated decay>. <Jens Lykke-Andersen> and colleagues recruited human <UPF proteins> to a β-globin reporter downstream of its termination <codon> and induced <RNA decay>. Pulse–chase measurements established decay, and dependence on <biological translation>, downstream positioning and functional UPF machinery connected the effect with surveillance rather than nonspecific destruction.<Human Upf proteins target an mRNA for nonsense-mediated decay when bound downstream of a termination codon by Jens Lykke-Andersen and others>[Reporter decay, position controls and UPF dependence]{ref} Recruitment can therefore supply an otherwise missing recognition signal. Together with association of <UPF proteins> with spliced <RNA>, the result helped connect a nuclear processing history to cytoplasmic interpretation of termination. It does not show that every natural surveillance substrate requires an <exon junction complex> or that tethered <UPF proteins> act without other factors.
A further use is to order the steps of <microRNA> silencing. Natural recognition involves a guide-loaded <Argonaute> complex and recruitment of <GW182>. Tethering <GW182> or its silencing domain bypasses guide–target pairing; mutating interaction surfaces or depleting downstream complexes then tests whether recruited <GW182> can promote repression and <deadenylation>. Biochemical interaction and functional analyses showed direct recruitment of cytoplasmic deadenylase complexes, including <CCR4-NOT complex>, by <GW182>.<GW182 proteins directly recruit cytoplasmic deadenylase complexes to miRNA targets by Joerg E. Braun and others>[Direct deadenylase interactions and silencing requirements]{ref} Combined evidence distinguishes deadenylase recruitment from a model in which tail shortening occurs only indirectly after translational arrest. Nevertheless, repression and decay can be coupled, and their order must be measured rather than inferred from one endpoint.
Domain deletions, catalytic mutants and interaction-defective variants make the approach particularly informative. If removing an RNA-binding domain destroys natural regulation but tethering restores it, that domain may chiefly mediate recruitment. If an effector-domain <mutation> remains inactive despite confirmed tethering, it identifies a post-recruitment requirement. Recruitment of an upstream component that fails when a downstream component is depleted provides an <epistasis> test; rescue with a depletion-resistant downstream factor strengthens the ordering. Expression and folding controls are essential because a deletion can destabilize the entire <protein>. Conversely, a tethered factor may simply recruit an endogenous partner, so sufficiency of a fusion does not establish autonomous catalytic action.
The strongest interpretation combines these manipulations with endogenous binding maps, <mutation> of natural <RNA> sites and physiological loss-of-function/rescue experiments. The resulting contribution is a modular view of regulation: <RNA> recognition chooses the substrate, while separable interaction or catalytic domains determine the outcome, and <RNA> position can change that outcome. \b[Tethering establishes conditional sufficiency after recruitment; native targeting and physiological necessity require complementary experiments.]
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