Which Equation Better Describes the Steady-State Concentration of a Protein

Which equation better describes the steady-state concentration of a protein (X) that is regulated by the simple binding of a transcription repressor (R) to its cis?-regulatory DNA sequences? β is the transcription rate, m is the translation rate, K is the association constant in the binding of R to the regulatory sequences, and τX is the mean lifetime of the protein X.

A) βmτX / (1 + K[R])
B) βmτX / (1 + [R])
C) βmτX K / (1 + K[R])
D) βmτX [R] / (1 + K[R])
E) βmτX [R] / (K + [R])