Translational inactivation of RNA function: discrimination against a subset of genomic transcripts during HBV nucleocapsid assembly.

Hepatitis B virus (HVB) is the prototype member of the hepadnaviridae, a family of small enveloped DNA viruses that replicate by reverse transcription. Assembly of replication-competent HBV nucleocapsids is based on specific interactions between the core protein, the product(s) of the P gene, and the RNA pregenome, which is marked for encapsidation by containing a sequence near its 5' end that acts in cis as an encapsidation signal. However, HBV produces several additional, almost identical, genomic transcripts that also bear the encapsidation sequence, but that are not encapsidated. The mechanism underlying this selection process has remained mysterious. Here we demonstrate that translating 80S ribosomes (but not scanning 40S ribosomal subunits) advancing into the encapsidation signal prevent its functioning. This finding reveals translational modulation of RNA function as a further regulatory mechanism employed by hepadnaviruses to utilize efficiently the restricted coding capacity of their extremely compact genome.