Final answer:
External stimuli can alter post-transcriptional control of gene expression by impacting mRNA stability and translation efficiency, most often through processes like phosphorylation of proteins related to gene regulation.
Step-by-step explanation:
External stimuli can alter post-transcriptional control of gene expression by modifying mRNA stability or translation efficiency. Environmental factors like stress, nutrients scarcity, heat, or ultraviolet light induce chemical changes that affect epigenetic accessibility, and thus gene expression. For instance, the phosphorylation of a protein can impact several processes including chromosomal accessibility, mRNA stabilization, translation efficiency, and post-translational modifications, leading to an upregulation or downregulation of specific proteins in response to the environment. These modifications do not change the DNA sequence itself but rather affect the structures and interactions of proteins involved in gene regulation.