The cya, crp, rho and nus gene products modulate the expression of a wide variety of bacterial and bacteriophage genes or operons. In order to understand the regulatory processes, we are studying the structure, expression and activity of these genes. We have previously shown that the protein products of the cya, crp, and rho genes are autogenously regulated. In CRP, cyclic AMP binding to the amino-termianl domain induces an allosteric transition which changes the DNA binding property of the carboxy domains. We have isolated mutants in the crp gene called crp*, which make CRP functional in the absence of cyclic AMP. The mutations responsible for the CRP* phenotype cause substitutions by amino acids with bulkier side chains in the D Alpha-helix of the protein's carboxy domain, near the hinge which connects the carboxy to the amino domain. Apparently, the mutant CRP*s have assumed a conformation that is normally evoked by cAMP binding. Our study defines precisely that amino acids and thus the Alpha-helicles which interact specifically to cause the allosteric shift. We have found that a NusA amber fragment is still functional for anti-termination when expressed from a multicopy plasmid. This suggests that it is the aminoterminal portion of NusA that is important for anti-termination. We have shown by pulse-labeling of RNA and by DNA-RNA hybridization, as well as by operon fusion analysis, that the autogenous regulation of rho is at the level of transcription. We have also found, by similar analysis, that cyclic AMP is a positive effector for rho gene expression, but also acts as a """"""""repressor"""""""" of rho mRNA translation. This opposite control of the rho gene by cyclic AMP would maintain a constant level of Rho in cell. We are currently studying the system in vitro.
