Conditional gene expression systems have developed into essential tools for the study of gene functions. However, their utility is often limited by the difficulty of identifying clonal cell lines, in which transgene control can be realized to its full potential. Here, we describe HeLa cell lines, in which we have identified—by functional analysis—genomic loci, from which the expression of transgenes can be tightly controlled via tetracycline-regulated expression. These loci can be re-targeted by recombinase-mediated cassette exchange. Upon exchange of the gene of interest, the resulting cell line exhibits the qualitative and quantitative properties of controlled transgene expression characteristic for the parent cell line. Moreover, by using an appropriate promoter, these cell lines express the tetracycline controlled transcription activator rtTA2-M2 uniformly throughout the entire cell population. The potential of this approach for functional genomics is highlighted by utilizing one of our master cell lines for the efficient microRNA-mediated knockdown of the endogenous human lamin A/C gene.

In the halotolerant sugar beet co-expression of V-ATPase and a vacuolar Na+/H(+)-antiporter provides a mechanism for vacuolar salt sequestration. To analyze salt-induced changes in the expression of the vacuolar H(+)-ATPase (V-ATPase) a partial cDNA of the proton-channel forming subunit c was cloned by RT-PCR. Southern blot analysis indicated a small gene family. In control plants transcript levels were high in roots and young growing leaves but low in fully expanded leaves. In mature leaves salt exposure (400 mM, 48 h) induced a strong increase in subunit c-mRNA. Transcripts for the catalytic subunit A followed a similar developmental and stress-modulated pattern, indicating a coordinate regulation of transcripts for both V-ATPase subunits. Concomittant with the mRNA increases the amount of V-ATPase protein increased as well.

Interferons control viral replication and the growth of some malignant tumors. Since systemic application may cause severe adverse effects, tissue-specific expression is an attractive alternative. Liver-directed interferon gene therapy offers promising applications such as chronic viral hepatitis B or C or hepatocellular carcinoma and thus needs testing in vivo in suitable animal models. We therefore used the Tet-On system to regulate gene expression in adenoviral vectors, and studied the effect of liver-specific and regulated interferon g expression in a mouse model of chronic hepatitis B virus (HBV) infection. In a first generation adenoviral vector, genes encoding for firefly luciferase and interferons a, b or g, respectively, were coexpressed under control of the bidirectional tetracycline-regulated promoter P tet bi. Liverspecific promoters driving expression of the reverse tetracycline controlled transactivator ensured local expression in the livers of HBV transgenic mice. Following gene transfer, we demonstrated low background, tight regulation and a 1000-fold induction of gene expression by doxycycline. Both genes within the bidirectional transcription unit were expressed simultaneously, and in a liver-specific fashion in cell culture and in living mice. Doxycycline-dependent interferon g expression effectively controlled HBV replication in mice, but did not eliminate HBV transcripts. This system will help to study the effects of local cytokine expression in mouse disease models in detail. Gene Therapy (2005) 12, 668-677.

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The improvement of safety and titer of retroviral vectors produced in standard retroviral packaging cell lines is hampered because production relies on uncontrollable vector integration events. The influences of chromosomal surroundings make it difficult to dissect the performance of a specific vector from the chromosomal surroundings of the respective integration site. Taking advantage of a technology that relies on the use of packaging cell lines with predefined integration sites, we have systematically evaluated the performance of several retroviral vectors. In two previously established modular packaging cell lines (Flp293A and 293 FLEX) with single, defined chromosomal integration sites, retroviral vectors were integrated by means of Flp-mediated site-specific recombination. Vectors that are distinguished by different long terminal repeat promoters were introduced in either the sense or reverse orientation. The results show that the promoter, viral vector orientation, and integration site are the main determinants of the titer. Furthermore, we exploited the viral production systems to evaluate read-through activity. Read-through is thought to be caused by inefficient termination of vector transcription and is inherent to the nature of retroviral vectors. We assessed the frequency of transduction of sequences flanking the retroviral vectors from both integration sites. The approach presented here provides a platform for systematic design and evaluation of the efficiency and safety of retroviral vectors optimized for a given producer cell line.

Cell extension growth in the mesocotyl tip of darkgrown Zea mays L. seedlings is dependent on vacuole enlargement and massive flux of ER and Golgi vesicles. Water flow into the expanding vacuole is driven by ion accumulation, which in turn is energized by the vacuolar H+-ATPase (V-ATPase). The V-ATPase energizes the secondary ion transport into the expanding vacuole. As light exposure leads to a strong inhibition of extension growth, the effect of light on transcript levels for subunits A and c of the V-ATPase was analyzed. Partial homologous cDNAs for subnnit A and two isoforms of snbunit c were cloned by RT-PCR. In dark-grown seedlings transcript levels for both subnnits were much higher in the growing mesocotyl tip than in the fully differentiated mesocotyl tissue. Only in the tip region did light exposure lead to a strong and coordinate down-regulation of both mRNAs whereas in the differentiated mesocotyl only a slight decrease was observed. The results indicate that expression of the 'housekeeping' V-type H +-ATPase is strongly regulated in response to growth rate. maize seedlings was chosen as a well defined experimental system. Here, elongation is the result of directional cell extension and is inhibited by light exposure [10][11][12]. In the etiolated seedling, cell division and elongation in the mesocotyl tip (5 mm) are dependent on intense traffic of ER and Golgi vesicles and vacuole expansion [13]. These processes are in turn dependent on luminal acidification by V-ATPase and, possibly, H---PPiase, which resides on the same membrane [14]. We have studied the expression of V-ATPase genes coding for the catalytic subunit A (part of the peripheral Vl-complex) and the proton channel forming subunit c (part of the membrane-integral Vo-complex) [1]. Transcript levels of both subunits were analyzed in the growing mesocotyl tip (0-5 mm) and the mesocotyl base (5-70 mm) of 6-day-old etiolated Zea mays seedlings before and after light exposure. The results indicate that expression of V-ATPase genes is strongly correlated with extension growth.

salt stress conditions which was assumed to be a proteolytic breakdown product of the catalytic subunit A [6].Preliminary results raised the question whether polypeptides Di and Ei are genuine subunits of the V-ATPase or proteolyric products of subunits A or B [7] which, however, remain attached to the V-ATPase holoenzyme and might affect holoenzyme stability [8]. Recently, sequence information has been obtained for subunit B of Hordeum vulgare [9] and M. crystal linum [10]. The alignment of the N-terminal sequence of the Oi polypeptide with the subunit B protein sequences indicates that Di may be derived from subunit B by proteolytic processing or breakdown. In agreement with this assumption an antiserum prepared against Di cross-reacts with subunit B and interferes with V-ATPase function as shown in this report. Materials and methods