Hyper-regulation of pyr gene expression in XXXX cells with slow ribosomes. Evidence for RNA polymerase pausing in vivo?

@entity526 @entity492 @entity17450 @entity856
@entity856 -modulated attenuation of transcription is involved in regulating the synthesis of @entity17450 in @entity526 . Thus, expression of two genes, pyrBI and pyrE, was shown to be under this type of control. The genes encode the two subunits of @entity492 transcarbamylase and orotate phosphoribosyltransferase respectively. The levels of these enzymes are inversely correlated with the intracellular concentration of @entity856 . Modulation of attenuation seems to be a consequence of the effect of @entity856 concentration on the mRNA chain growth rate. Reducing the @entity856 pool retards RNA polymerase movement. Mechanistically this will couple the ribosomes translating a leader peptide gene more tightly to the elongating RNA polymerase. The ribosomes will then be more prone to prevent the folding of the mRNA chains into terminating hairpin structures when RNA polymerase is at the attenuator and has to decide whether transcription should terminate or continue into the structural genes. This paper described a study of pyrBI and pyrE gene regulation in cells where the ribosomes move slowly as a result of mutation in rpsL. It appears that expression of the two genes is hyper-regulated by the @entity856 pool in this type of cells. Furthermore, the attenuator model can only account for the results if it is assumed that @entity856 -concentration-dependent pausing of transcription occurs in vivo in the two pyr gene leaders such that RNA polymerase waits for the coupled ribosomes before transcribing into the attenuator regions.
