Department of Human Genetics, University of Utah, Salt Lake City 84112.
Gene 10 of bacteriophage T7, which encodes the most abundant capsid protein, has two products: a major product, 10A (36 kDa), and a minor product, 10B (41 kDa). 10B is produced by frameshifting into the -1 frame near the end of the 10A coding frame and is incorporated into the capsid. The frameshift occurs at a frequency of about 10% and is conserved in bacteriophage T3. This study shows that sequences important to frameshifting include the originally proposed frameshift site, consisting of overlapping phenylalanine codons and the 3' noncoding region that includes the transcriptional terminator over 200 bases downstream of the frameshift site. The frameshift occurs at the overlapping phenylalanine codons as determined from peptide sequencing data. Complementation studies show that there is only a very weak phenotype associated with phage infections in which there is no 10A frameshifting. Capsids from such infections are devoid of 10B and are as stable as wild-type capsids.