Answer:
Strain A is subject to posttranscriptional gene silencing, whereas Strain B is subject to transcriptional gene silencing
Step-by-step 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).