Transcription of bacteriophage Mu. II. Transcription of the repressor gene

Using pBR322 as a vector, three plasmids were constructed, pGP2, pGP3, and pGP7, containing respectively 5, 100, 700-950, and 1,000 base pairs derived from the immunity end of bacteriophage Mu. All three plasmids contain a functional repressor gene coding for a thermosensitive repressor. RNAs produced when the DNA of these plasmids was used as template in in vitro RNA synthesis, were analysed by hybridization to the DNA of several lambda pMu transducing phages. In spite of the differences in length of the Mu fragments all three plasmids show the same amount of Mu specific l-strand transcription. Since the repressor gene comprises at least 70% of the Mu fragments of pGP3 and pGP7, these results indicate that the repressor gene c of bacteriophage Mu is transcribed on the l-strand. Analysis of in vivo RNA from cells harboring the plasmids pGP2, pGP3, or pGP7 also indicates that the repressor gene of phage Mu is transcribed on the l-strand, as all Mu-specific RNA extracted from these cells at 28 degrees C hybridizes with the l-strand of the first 3,100 basepairs from the Mu immunity end.

Invertible DNA determines host specificity of bacteriophage mu

The function of the invertible G region of bacteriophage Mu is apparently to confer different host specificities on Mu. Two products of genes S and U, situated in the G region are not needed for the infectivity of Mu G(-) particles. In the Mu G(-) phage the S gene product and the 21-K polypeptide, presumably the product of gene U, are missing. Instead, two other polypeptides with different molecular weights are observed.

Expression of the cloned uvrB gene of Escherichia coli: dependency on nonsense suppressors

Recombinant plasmid pNP5, consisting of plasmid pMB9 on which the uvrB gene is cloned, fully complements for the defects due to chromosomal uvrB mutations in the presence of the amber suppressor sup-6 or supF. Correndonuclease II activity was also completely restored in in UvrB strains containing both plasmid pNP5 and amber suppressor sup-6, as compared with the parental UvrB+ strain. It is shown that the amber mutation which interferes with the expression of the cloned uvrB gene is located outside this gene. Apparently, the amber mutation exerts a polar effect on uvrB expression that is relieved by sup-6 or supF. Introduction of a rho mutation into suppressor-free UvrB strains, harboring pNP5, did not relieve the polarity caused by the amber mutation.

Expression of the cloned uvrB gene of Escherichia coli: mode of transcription and orientation

The Escherichia coli uvrB gene, located on a 1.5-megadalton EcoRI (fragment F, derived from transducing phage lambda b2att2 [lambda b2cI857intam6 delta (bioAB)bio-FCD+uvrB+], has been cloned in the unique EcoRI site of several "relaxed" plasmids, i.e., pMB9, pBR322, and pBH20 (= ;BR322, including the lac regulatory elements [K. Itakura, T. Hirose, R. Crea, A. D. Riggs, H. L. Heyneker, F. Bolivar, and H. W. Boyer, Science 198:1056--1063, 1977]y. Expression of the uvrB gene, both on pMB9 and on pBH20, occurs only when fragment F has one particular orientation. Cloning of this fragment on pBR322 in either orientation does not allow expression of the uvrB gene. Transcription of this gene on pNP5 ( = pMB9 uvrB) is shown to be dependent on a pMB9 promotor that is located on a 0.22-megadalton EcoRI-HindIII fragment. Using plasmid pBH20 as a vector, we could demonstrate that expression of the uvrB gene is under control of the lac promotor-operator region. From deoxyribonucleic acid-deoxyribonucleic acid hybridization experiments with lambda pgal8 deoxyribonucleic acid and restriction fragments of pNP5 deoxyribonucleic acid it could be shown that the uvrB gene is transcribed clockwise on the chromosome.

Expression of the uvrB gene of Escherichia coli: in vitro construction of a pMB9 uvrB plasmid

Bacteriophage lambdab2att2 [lambdab2cI857intam6(deltabioAB)bioFCD+uvrB+phr+] codes for a function(s) that confers UV resistance (Uvr+) and reactivation of irradiated phage (Hcr+) to an Uvr-Hcr-Escherichia coli strain. It was demonstrated that these functions are expressed under the control of bacterial regulatory elements located on lambdab2att2 DNA. The location of the E. coli uvrB gene on the DNA of this transducing phage was established by heteroduplex and restriction-enzyme analyses. Recombinant DNA molecules were constructed in vitro from plasmid pMB9 (Tcr), as the vector, and an EcoRI fragment (Eco-RI-F) of lambdab2att2 DNA. The resulting plasmid, designated pNP5, has a molecular weight of 5.1 X 10(6) and replicates in a relaxed fashion. Transformation of E. coli uvrB with plasmid pNP5 resulted in clones that are Uvr+ Tcr. Irradiation of bacteria transformed with plasmid pNP5 with low UV doses revealed a complete restoratation of the Uvr+ phenotype by the presence of the cloned EcoRI-F DNA, while only a partial restoration was observed after irradiation with high UV doses. Likewise, the Hcr+ character was also partially restored due to the presence of pNP5. No correlation was found between the acquired Uvr+, Hcr+ properties, and the presence of correndonuclease II activity in an extract of bacteria that harbor plasmid pNP5.

Defective prophages of bacteriophage Mu

A method is described for the isolation of thermoinducible defective Mu lysogens. Four of these defective lysogens were studied more extensively. By marker-rescue experiments it was shown that the strain harbouring the smallest defective prophage contains the immunity gene cts and the genes A and B; the strain with the largest defective prophage still contains all the known essential genes of Mu, A to S (see Fig. 1). After induction at 43 degrees C all the defective lysogens are killed, whereas no lysis occurs. Although in all the thermoinducible defective lysogens the A and B gene products could be demonstrated by complementation, these gene products are not responsible for the killing of the host, suggesting the presence of another unknown early gene product of Mu. The level of complementation of a mutation in gene A is reduced by the presence in the cell of another defective Mu prophage containing the "G" beta part of Mu. This effect on A gene complementation is markedly enhanced when the defective prophage, containing the "G" beta part, is located on an episome instead of on the chromosome. Complementation of late genes by a defective prophage located on the chromosome, is extremely low or undetectable. A stimulation of complementation by a factor of 10 to 40 was found when the same defective prophage was situated on a F' factor. A possible explanation for this "episome" effect will be discussed.

Transcription of bacteriophage mu. An analysis of the transcription pattern in the early phase of phage development

It has previously been shown that the transcription of Mu is asymmetric and takes place on the heavy DNA strand (Bade, 1972; Wijffelman et al., 1974). The direction of transcription of Mu has now been determined by RNA-DNA hybridizations between purified Mu-RNA and the separated strands of lambda-Mu hybrid phages. The direction of transcription is from the c-gene (immunity gene) end of the heavy strand to the beta-end (immunity distal end) (Fig. 1). Thermo-inducible, defective Mu lysogens, in which the prophage is deleted from the beta-end, have a normal early transcription pattern, but the increase of RNA at later times is absent. A defective lysogen, which contains only the immunity gene c and the genes A and B, still has an early transcription pattern similar to that of the wild-type. Therefore, we conclude that the early RNA is transcribed from that region of the Mu genome. The early Mu-RNA synthesis is negatively regulated with a minimum rate of transcription at 9 minutes after induction. Before the onset of the late RNA synthesis, at about 22 minutes there is a rather long period in which the rate of Mu-RNA synthesis slowly increases. Using DNA strands of lambda-Mu hybrids which contain only that part of the Mu-DNA on which the early RNA synthesis takes place, we have determined that during the first half in the intermediate phase only early genes are transcribed. The amount of Mu-RNA synthesized by a Mu prophage carrying the X-mutation, which influences the excision of Mu, is greatly reduced. Negative regulation of early transcription occurs normally in this mutant.