None of the references described herein are admitted to be prior art to the claimed invention.
A target nucleic acid sequence can be detected by various methods using nucleic acid probes designed to preferentially hybridize to the target sequence over other sequences that may be present in a sample. Examples of target sequences include sequences that may be initially present in a sample, or produced as part of an amplification procedure, such as a sequence characteristic of a microorganism, a virus, a plant gene, or an animal gene such as a human gene. A reporter sequence which is produced as part of a detection method in the presence of a target sequence, but which has a sequence that is not dependent on the target sequence, can also be detected.
Hybridization of probes to target nucleic acid sequences can form detectable probe:target duplexes under appropriate conditions. Detection of such duplexes is facilitated using a labeled probe. Different techniques are available to reduce background due to signal from labeled probes not hybridized to a target sequence. Such techniques include using a physical separation step, a label preferentially altered in a probe:target duplex versus an unhybridized probe, and/or interacting labels.
Interacting labels are two or more labels which cooperate when in close proximity to one another to produce a signal which is different from a signal produced from such labels when they are farther apart so that their cooperation is diminished. The labels may be associated with one or more molecular entities. Detection systems can be designed such that the labels interact either in the presence of a target sequence or in the absence of a target sequence.
Taub et al., U.S. Pat. No. 4,820,630 describes interacting labels present on two different nucleic acid molecules cooperating to produce a detectable signal in the presence of a target nucleic acid sequence. Hybridization of both molecules to the target sequence brings the labels into close proximity so that they can cooperate to produce a signal different from labels not cooperating in close proximity.
Morrison, European Application Number 87300195.2, Publication Number 0 232 967, describes a detection system involving a reagent made up of two complementary nucleic acid probes. One of the complementary probes contains a first label, and the other complementary probe contains a second label. The first and the second labels can interact with each other. Formation of a complex between the target sequence and one of the two complementary probes changes the interaction between the two labels.
Lizardi et al., U.S. Pat. Nos. 5,118,801 and 5,312,728, describes a nucleic acid probe containing a target complementary sequence flanked by “switch” sequences that are complementary to each other. In the absence of a target sequence, the switch sequences are hybridized together. In the presence of a target sequence the probe hybridizes to the target sequence, mechanically separating the switch sequences and thereby producing an “open switch”. The state of the switch sequence, whether open or closed, is indicated to be useful for selectively generating a detectable signal if the probe is hybridized to the target sequence.
Lizardi et al., International Application Number PCT/US94/13415, International Publication WO 95/13399, describes a “unitary” hybridization probe. The probe contains a target complementary sequence, an affinity pair holding the probe in a closed conformation in the absence of target sequence, and a label pair that interacts when the probe is in a closed conformation. Hybridization of the probe to the target sequence shifts the probe to an open conformation, which reduces the interaction between the label pair.
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