86.在細胞中，管家基因 （housekeeping genes）的種類繁多，但個別基因的表現量不同。下列何者是調控管家 基因表現的最主要機制？
(A)各基因產物合成率一致，但其降解速率各異
(B)各基因所轉錄的mRNA量一致，但其轉譯的速率各異
(C)各基因的啟動子對RNA聚合酶的親和力各異
(D)各基因表現的程度受到不同的誘導或抑止因子所調節

Genes  for  products  that  are  required  at  all  times,  such  as  those  for  the enzymes  of  central  metabolic  pathways,  are  expressed  at  a  more  or  less constant level in virtually every cell of a species or organism. Such genes are often referred to as housekeeping genes.

For  other  gene  products,  cellular  levels  rise  and  fall  in  response  to molecular  signals;  this  is  regulated  gene  expression.  Gene  products  that increase  in  concentration  under  particular  molecular  circumstances  are referred  to  as  inducible;  the  process  of  increasing  their  expression  is induction.  The  expression  of  many  of  the  genes  encoding  DNA  repair enzymes,  for  example,  is  induced  by  a  system  of  regulatory  proteins  that responds  to  high  levels  of  DNA  damage.  Conversely,  gene  products  that decrease in concentration in response to a molecular signal are referred to as repressible, and the process is called repression. For example, in bacteria, ample supplies of tryptophan lead to repression of the genes for the enzymes that catalyze tryptophan biosynthesis.

Although  housekeeping  genes  are  expressed  constitutively,  the  cellular concentrations of the proteins they encode vary widely. For these genes, the RNA  polymerase–promoter  interaction  strongly  influences  the  rate  of transcription  initiation;  differences  in  promoter  sequence  allow  the  cell  to synthesize the appropriate level of each housekeeping gene product.