There are two strains of E. coli, let's call them strain A and B. Both are his- mutants. A his-
mutant is unable to synthesize the amino acid
histidine. Both strains are unable to grow on
chemically defined media if the media is not
supplemented with histidine. Careful analysis of
strain A demonstrated that the mRNA for
histidine genes is transcribed but not translated.
No histidine mRNA was present in the cytosol of
strain B. How is this possible?

Strain A is subject to posttranscriptional gene silencing, whereas Strain B is subject to transcriptional gene silencing

Explanation:

During transcription, a segment of DNA (e.g., a gene) is used as a template to synthesize a complementary RNA molecule, typically a messenger RNA (mRNA) molecule, which is in turn used as a template to synthesize a polypeptide chain (i.e., a protein) by a process known as translation. In consequence, gene expression involves two different processes (i.e., transcription and translation), by which genetic information contained in DNA flows from genes to proteins. Transcriptional silencing refers to the phenomena by which the transcription of one or more genes (in this case, the genes necessary for histidine biosynthesis) are repressed or not, dependent only on the gene's chromosomal location, whereas posttranscriptional silencing occurs when transcripts do not accumulate in spite of continuous mRNA transcription. In this case, mRNA synthesis is expected to occur in strain A his-mutants (i.e., strain A is subject to posttranscriptional gene silencing), whereas mRNA synthesis is not expected to occur in strain B his-mutants (i.e., strain B is subject to transcriptional gene silencing).