Provided herein are systems and methods for nucleotide incorporation reactions. The systems comprise polymerases having altered nucleotide incorporation kinetics and are linked to an energy transfer donor moiety, and nucleotide molecules linked with at least one energy transfer acceptor moiety. The donor and acceptor moieties undergo energy transfer when the polymerase and nucleotide are proximal to each other during nucleotide binding and/or nucleotide incorporation. As the donor and acceptor moieties undergo energy transfer, they generate an energy transfer signal which can be associated with nucleotide binding or incorporation. Detecting a time sequence of the generated signals, or the change in the signals, can be used to determine the order of the incorporated nucleotides, and can therefore be used to deduce the sequence of the target molecule.