The nature of uridine phosphorylase constitutive synthesis was studied in the rho15(ts) mutant strain of Escherichia coli. The rho15 mutation causes the 8-10 fold increase in uridine phosphorylase activity under conditions of both induction of enzyme synthesis by cytidine and complete inhibition of the udpP promoter activity in the crp background. These data indicate that regulation of the udp gene which is controlled by the cytR repressor protein and by cyclic AMP -- CRP complex, is disturbed in the presence of the mutated rho factor. Introduction of the rho15 mutation into the udpP1 and udpP18 promoter mutants which are characterized by cytR and (or) CRP independent expression of the udp gene, leads to 2 fold reduction in uridine phosphorylase activity. From this, it may be concluded that the presence in bacteria of the rho15 mutation prevents transcription initiation from the intact udpP+ promoter and also leads to udpP1 and udpP18 mutant promoters inhibition. On the basis of these data, it is proposed that the effect of rho15 mutation on the udp gene expression is rather due to read-through transcription from an upstream highly efficient foreign promoter, than to relief of attenuation within the udp gene regulatory region. The uridine phosphorylase activity under control of this foreign promoter, i.e. in the rho15 genome, is reduced 2-3 fold when bacteria are grown on the minimal medium supplemented with L-methionine or casaamino acids. Based on these dat, it is suggested that increased udp gene expression in the rho15 background is due to read-through transcription, possibly, from the promoter of the neighbouring metE gene.