FIG. 4.

The IC45-44 regions of T4 and RB69 (A) and plasmid-mediated expression of genes 45, 44, and 62 (B). The T4 sequence has been reported previously (23–25; see also reference 8). The RB69 sequence was obtained from four different subclones that were derived from the RB69 λZAPII genomic library and confirmed by determinations on independent clones that were obtained after PCR amplification of genomic DNA. The alignment shown in panel A was purposely configured to depict the highest degree of similarity (51% [Table 1]) between the two sequences. Note that the two intercistronic regions differ in size (50 bp for T4 and 75 bp for RB69). The grey-shaded sequences constitute parts of open reading frames. The black-shaded sequences (underlined by arrows) constitute the stem portions of predicted mRNA secondary structures. The T4 RNA structure functions as part of a transcription termination signal (8). In the experiment for panel B, comparable DNA segments from the T4 and RB69 genomes were inserted in λpLN and λpL vectors and then introduced into E. coli NapIV for measurements of gene expression as described in Materials and Methods. The arrows on the SDS-PAGE autoradiogram indicate positions of the ³⁵S-labeled plasmid-derived phage proteins. A 7.5%-10%-12.5% step gradient gel was used.