Interaction of rho factor with bacteriophage lambda cro gene transcripts

Rho protein is responsible for termination of transcription of the cro gene of bacteriophage lambda. Since rho is known to interact with the RNA whose synthesis is being terminated, we measured the specificity and strength of binding of rho to isolated cro transcripts, using a nitrocellulose filter retention assay. The association constant (K alpha) for the binding of rho to a 372-nucleotide cro transcript was determined to be 7 +/- 2 X 10(8) M-1 at 37 degrees C and about 20-fold less at 4 degrees C. Although NTP cleavage is required for rho activity, the presence of ATP did not alter the K alpha. Rho bound less tightly (K alpha less than 10(8) M-1) to partial cro transcripts smaller than 290 nucleotides and had very little affinity (K alpha less than 10(6) M-1) for lambda 4 S RNA, lambda 6 S RNA, and partial cro transcripts smaller than 160 nucleotides. In contrast, cro transcripts as short as 100 nucleotides bound if guanosine residues were replaced with inosine. In addition, rho bound readily to 3‘ fragments of cro RNA that had 85 or more residues. A common feature of the RNA molecules that bind tightly to rho protein is that they have a stretch of at least 85 nucleotides with relatively few (less than 14%) guanosine residues. Such a segment is thus likely to be largely single-stranded. These results suggest that the binding of rho to lambda cro mRNA is dependent on a 3‘ terminal segment that has those properties.