Chain initiation in a polycistronic message: sequential versus simultaneous derepression of the enzymes for histidine biosynthesis in Salmonella typhimurium.

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Abstract
L-serine, L-methionine and adenine shift the mode of derepres-sion from sequential to simultaneous by increasing the 1-C pool of the cell. The ability of trimethoprim to prevent the effect of these compounds on the mode of derepression was investigated to demonstrate that the effect on derepression is prevented when utilization of 1-C fragments in impaired. The latter compounds are not able to shift the mode of derepression from sequential to simultaneous in the presence of the inhibitor of dihydrofolate reductase. The manner in which the polycistronic message of the histidine operon is translated in-vivo depends upon the capacity of the cell to initiate polypeptide chains. The amount of NlO-formyltetrahydrofolate available (presumably for formyl-ation of methionyl-tRNA) determines whether chain initiation occurs only at the operator end or at multiple sites along the polycistronic messenger-RNA. In those mutants that synthesize phosphoribosyl-4-amino-5-jmidazolecarboxamide-(AIC) via the histidine pathway, the polycistronic histidine message is initiated only at the operator end producing the sequential pattern of derepression; whereas translation of this message is initiated at multiple points, producing the simultaneous pattern of derepression in those mutants that do not synthesize phosphoribosyl-AIC via the histidine pathway. The manner in which phosphoribosyl-AIC produces the sequential pattern of derepression is indirect. The conversion of phosphoribosyl-AIC to formylphosphori-bosyl-AIC produces the sequential pattern of derepression by limiting the formylating capacity of the cell for polypeptide chain initiation. A limitation of the formylating capacity of the cell produces the sequential mode of derepression. Polypeptide chain initiation occurs preferentially at the operator end of the message when the formylating capacity is limiting. Since translation of the histidine message can be initiated at multiple sites only when formyl groups are not limiting, formyl groups may be utilized in the initiation reactions at these sites. N-formyl-methionyl-tRNA is required for polypeptide chain initiation at each cis-tron. N-formylmethionyl-tRNA is required for amino acid incorporation into all the proteins programed by the polycistronic f2 RNA. N-formylmethionine is the amino-terminal amino acid for all the proteins programed by the polycistronic MS2 RNA. The codons which specify N-formylmethionyl-tRNA when present at the 5[image]-terminus do not do so when present at internal positions, indicating that punctuation exists in the polycistronic histidine message in-vivo allowing such codons to specify N-formylmethionyl-tRNA despite their internal positions.