[Structure and stability of cointegrating plasmids mediated by the IS1 elements in transposon delta Tn9']. / Struktura i stabil'nost' kointegratnykh plazmid, oposredovannykh élementami IS1 v sostave transpozona delta Tn9'.

In order to elucidate the structural features of the transposon Tn9', representative of the Tn9 family, which define the ability of the transposon to produce unstable cointegrates, we have obtained a derivative of this transposon carrying a deletion in its central region. The deletion in the obtained transposon delta Tn9' covers a DNA segment of about 50 bp in length, occupying the most distal position in relation to the cat gene, at its junction with the right copy of the IS1. The structure and stability of the IS1/delta Tn9'-mediated cointegrates between the plasmids pDK57.1 (pBR322::delta Tn9') and pRP3.1, a deletion derivative of RP1, have been studied. The three types of cointegrates were found. Those of the type I are predominantly formed, due to the left copy of the IS1 which in delta Tn9' occupies proximal position to the promoter of the cat gene. These cointegrates contain three copies of IS1 and are of high stability. The cointegrates of the type II contain two entire copies of delta Tn9' (i.e. four copies of IS1) as well as the structures of the type II, representing the cointegrate equivalent of inverse transposition and also containing four copies of IS1. Cointegrates of the type II and III dissociate efficiently in the rec+ cells but, in contrast to the cointegrates mediated by the original transposon Tn9', are unable to dissociate efficiently in the recA- cells. It was concluded that a DNA segment in the central region of Tn9' may be essential for the expression of the IS1-specific resolvase encoded by the right copy of IS1.

[Enhancement of expression of Escherichia coli uridine phosphorylase gene as a result of duplication]. / Usilenie ekspressii gena uridinfosforilazy (udp) Escherichia coli v rezul'tate duplikatsii.

The thymine-requiring (thy) Escherichia coli strains normally convert thymine to thymidine through the action of thymidine phosphorylase, an enzyme, whose synthesis is directed by tpp gene of the deo operon. Selection for efficient thymine utilization in the thy mutants carrying a deletion in the tpp gene allows to obtain clones with constitutive synthesis of an alternative enzyme, uridine phosphorylase coded by the udp gene. Such clones are usually formed, due to "promoter-up" constitutive udpP mutants. Further selection for increased expression of the udp gene was performed on the background of udpP1 and udpP18 "promoter-up" constitutive mutants isolated previously. Several lines of evidence obtained by using transposon insertions suggest that the increased synthesis of uridine phosphorylase in some newly isolated mutants is due to duplications of the udp gene. In two mutants the adjacent metE gene is involved in duplications simultaneously with the udp gene.

[Regulation of the activity of the Escherichia coli K-12 uridine phosphorylase gene. I. Mapping of the structural gene mutations and a determination of the direction of transcription]. / Reguliatsiia aktivnosti gena uridinfosforilazy Escherichia coli K-12. Soobshchenie I. Kartirovanie mutatsii strukturnogo gena i operedelenie napravleniia transkriptsii.

A fine genetic mapping of some point and deletion mutations for uridine phosphorylase (udp) gene, including deletions covering simultaneously the udp and the adjacent metE gene was carried out. Deletions of the latter type do not recombine with the udp promoter mutations isolated previously (Mironov, Sukhodolets 1979) thus suggesting that the udp promoter is situated at the site adjoining metE.

[Effect of mutations for adenylate cyclase (cya) and the cyclic adenosine monophosphate receptor protein (cgp) on the gene expression of nucleoside catabolism in Escherichia coli]. / Vliianie mutatsii po adenilattsiklaze (cya) i belku-retseptoru tsiklicheskogo adenozinmonofosfata (cgp) na vyrazhenie genov katabolizma nukleozidov u Escherichia coli.

The effect of cya and crp mutations on the expression of the activity of nucleoside catabolizing genes has been studied in Escherichia coli. It is found that cya and crp mutants lose their ability to grow on nucleosides as carbon sources in spite of the preservation of the basal levels of nucleoside catabolizing enzymes, found in cell-free extracts of cya and crp mutants. It is shown that cya and crp mutations completely release the influence of the regulatory gene cytR on the activity of uridine phosphorylase (udp gene) and thymidine phosphorylase (tpp gene). On this ground it is assumed that the cytR gene product acts at the level of promotors of the corresponding structural genes, causing their insensitivity to the positive action of cAMP--CRP complex. The same data concerning the effect of cya and crp mutations on cytR regulation have been reported [8], but these authors favoured the hypothesis that the cytR gene product is a repressor protein, which binds to the specific operator.