Q beta replicase containing altered forms of ribosomal protein S1.

Transcription of Q beta RNA replicase requires that the host-encoded Escherichia coli ribosomal protein S1 be present as a subunit of the replicase. To determine whether the activities of S1 in protein synthesis are operational in Q beta RNA transcription, we formed altered replicase enzymes by reconstituting replicase lacking S1 (R(-S1)) with modified S1 species whose properties in nucleic acid binding and protein synthesis had been previously established. S1 derivatized with N-ethylmaleimide reconstitutes a modified replicase that is 81% as active as replicase reconstituted with unmodified S1. S1 derivatized with N-ethylmaleimide and unmodified S1 bind with comparable affinity to R(-S1) (1 X 10(8) M-1). These results indicate that the helix-unwinding properties of S1, which are known to be inactivated by N-ethylmaleimide modification, are not required for Q beta RNA transcription and that the sulfhydryl-derivatized region of S1 is not utilized in replicase subunit contacts. In contrast with its established ability to replace E. coli S1 on the ribosome, Caulobacter crescentus S1 does not reconstitute Q beta RNA transcription activity when added to R(-S1). Our results suggest this inactivity may be due to poor binding to R(-S1).