Requirement of A-A-U-A-A-A and adjacent downstream sequences for SV40 early polyadenylation

Useful Bicistronic Reporter System for Studying Poly(A) Site-Defining cis Elements and Regulation of Alternative Polyadenylation

The link between polyadenylation (pA) and various biological, behavioral, and pathological events of eukaryotes underlines the need to develop in vivo polyadenylation assay methods for characterization of the cis-acting elements, trans-acting factors and environmental stimuli that affect polyadenylation efficiency and/or relative usage of two alternative polyadenylation (APA) sites. The current protein-based CAT or luciferase reporter systems can measure the polyadenylation efficiency of a single pA site or candidate cis element but not the choice of two APA sites. To address this issue, we developed a set of four new bicistronic reporter vectors that harbor either two luciferase or fluorescence protein open reading frames connected with one Internal Ribosome Entry Site (IRES). Transfection of single or dual insertion constructs of these vectors into mammalian cells demonstrated that they could be utilized not only to quantify the strength of a single candidate pA site or cis element, but also to accurately measure the relative usage of two APA sites at both the mRNA (qRT-PCR) and protein levels. This represents the first reporter system that can study polyadenylation efficiency of a single pA site or element and regulation of two APA sites at both the mRNA and protein levels.

Mechanisms of translational repression of the Smcp mRNA in round spermatids

The protamine 1 (Prm1) and sperm mitochondria-associated, cysteine-rich protein (Smcp) mRNAs exemplify a widespread pattern of mRNA-specific regulation of mRNA translation in post-meiotic spermatogenic cells, spermatids. Both mRNAs are transcribed and initially stored in free-mRNPs in early spermatids, and translated on polysomes in late spermatids. In this study, we demonstrate that the 5' and 3'-UTRs and the 3' terminus of the Smcp 3'-UTR are required for normal repression of the Smcp mRNA in transgenic mice. RNA affinity chromatography and mass spectrometry sequencing identified Y-box protein 2 (YBX2/MSY2) as the major protein that interacts with the 3' terminus of the Smcp 3'-UTR and a Y-box recognition sequence, GCCACCU, in the translation control element that is necessary for Prm1 mRNA repression. Depletion of YBX2 in Ybx2-null mice prematurely activates Prm1 and Smcp mRNA translation in early spermatids. Fluorescent in situ hybridization reveals that the Smcp intron, the Smcp mRNA, and both Smcp-Gfp transgenic mRNAs are strongly concentrated in the chromatoid body, and that theYbx2-null mutation does not eliminate the Smcp mRNA from the chromatoid body. This and previous findings suggest that the Smcp pre-mRNA is spliced and associates with YBX2 in the chromatoid body, and that repressed free-mRNPs are stored in the general cytoplasm. As YBX2 is the predominant protein in testis free-mRNPs, it likely represses many mRNAs in early spermatids. The mechanisms by which YBX2 represses the Smcp and Prm1 mRNAs are relevant to reproductive medicine because mutations in the human YBX2 gene correlate with abnormal protamine expression and male infertility.

Differences in polyadenylation site choice between somatic and male germ cells

BACKGROUND

We have previously noted that there were differences in somatic and male germ cell polyadenylation site choices. First, male germ cells showed a lower incidence of the sequence AAUAAA (an important element for somatic polyadenylation site choice) near the polyadenylation site choice. Second, the polyadenylation sites chosen in male germ cells tended to be nearer the 5' end of the mRNA than those chosen in somatic cells. Finally, a number of mRNAs used a different polyadenylation site in male germ cells than in somatic cells. These differences suggested that male germ cell-specific polyadenylation sites may be poor substrates for polyadenylation in somatic cells. We therefore hypothesized that male germ cell-specific polyadenylation sites would be inefficiently used in somatic cells.

RESULTS

We tested whether pre-mRNA sequences surrounding male germ cell-specific polyadenylation sites (polyadenylation cassettes) could be used to direct polyadenylation efficiently in somatic cells. To do this, we developed a luciferase reporter system in which luciferase activity correlated with polyadenylation efficiency. We showed that in somatic cells, somatic polyadenylation cassettes were efficiently polyadenylated, while male germ cell-specific polyadenylation cassettes were not. We also developed a sensitive, 3' RACE-based assay to analyze polyadenylation site choice. Using this assay, we demonstrated that male germ cell-specific polyadenylation cassettes were not polyadenylated at the expected site in somatic cells, but rather at aberrant sites upstream of the sites used in male germ cells. Finally, mutation of the male germ cell-specific poly(A) signal to a somatic poly(A) signal resulted in more efficient polyadenylation in somatic cells.

CONCLUSION

These data suggest that regulated polyadenylation site choice of male germ cell-specific polyadenylation sites requires one or more factors that are absent from somatic cells.

The ability of the HIV-1 AAUAAA signal to bind polyadenylation factors is controlled by local RNA structure

The 5' and 3' ends of HIV-1 transcripts are identical in sequence. This repeat region (R) folds a stem-loop structure that is termed the poly(A) hairpin because it contains polyadenylation or poly(A) signals: the AAUAAA hexamer motif, the cleavage site and part of the GU-rich downstream element. Obviously, HIV-1 gene expression necessitates differential regulation of the two poly(A) sites. Previous transfection experiments indicated that the wild-type poly(A) hairpin is slightly inhibitory to the process of polyadenylation, and further stabilization of the hairpin inhibited polyadenylation completely. In this study, we tested wild-type and mutant transcripts with poly(A) hairpin structures of differing thermodynamic stabilities for the in vitro binding of polyadenylation factors. Mutant transcripts with a destabilized hairpin efficiently bound the polyadenylation factors, which were provided either as purified proteins or as nuclear extract. The RNA mutant with a stabilized hairpin did not form this 'poly(A) complex'. Additional mutations that repair the stability of this hairpin restored the binding capacity. Thus, an inverse correlation was measured between the stability of the poly(A) hairpin and its ability to interact with polyadenylation factors. The wild-type HIV-1 transcript bound the polyadenylation factors suboptimally, but full activity was obtained in the presence of the USE enhancer element that is uniquely present upstream of the 3' poly(A) site. We also found that sequences of the HIV-1 leader, which are uniquely present downstream of the 5' poly(A) site, inhibit formation of the poly(A) complex. This inhibition could not be ascribed to a specific leader sequence, as we measured a gradual loss of complex formation with increasing leader length. We will discuss the regulatory role of RNA structure and the repressive effect of leader sequences in the context of differential HIV-1 polyadenylation.

Trans-splicing and alternative-tandem-cis-splicing: two ways by which mammalian cells generate a truncated SV40 T-antigen

The early SV40 BstXI-BamHI (Bst/Bam) DNA fragment encodes exclusively for the second exon of the large T-antigen and contains the intact small t-antigen intron. Rat cells transformed by the p14T, a construct that carries the Bst/Bam DNA fragment as a tail-to-head tandem duplication, synthesize a truncated T-antigen (T1-antigen) without having a direct equivalent at the DNA level. Formation of the T1-mRNA occurs by means of two distinct mechanisms: alternative-tandem-cis-splicing and trans-splicing. To generate the T1-mRNA the cells utilize a cryptic 5' splice site, located within the second exon of the large T-antigen and the regular small t-antigen 3' splice site. Since these splice sites are in an inverted order two Bst/Bam transcripts are required to generate one T1-mRNA molecule. For alternative-tandem-cis-splicing the cells utilize a 4.4 kb pre-mRNA that contains the sequence of the entire Bst/Bam tandem repeat. The proximal Bst/Bam segment provides the 5' donor splice site and the distal segment the 3' acceptor site. This requires that the pre-mRNA not be cleaved after the RNA polymerase II has passed the polyadenylation signal of the proximal Bst/Bam DNA segment. Synthesis of the 4.4 kb pre-mRNA was demonstrable by RT-PCR but not by Northern blot analysis. For trans-splicing, the cells utilize two separate pre-mRNA molecules. One transcript provides the cryptic 5' splice donor site and the other the 3' splice acceptor site. To demonstrate this a three base pair deletion was introduced into the proximal Bst/Bam segment of the p14T DNA (p14Tdelta-3) as a marker, destroying the recognition site for Pf/MI restriction enzyme. This deletion allowed the differentiation between the proximal and distal Bst/Bam segment. RT-PCR analysis and DNA sequencing confirmed that the p14Tdelta-3 transformed cells generate the T1-mRNA by intra- and inter-molecular RNA splicing.

Effect of deletions in the cauliflower mosaic virus polyadenylation sequence on the choice of the polyadenylation sites in tobacco protoplasts

Deletions were made in the cauliflower mosaic virus polyadenylation sequence which was cloned downstream of the beta-glucuronidase gene (gus). The populations of mRNAs generated in tobacco mesophyll protoplasts by transient expression with the various constructs were analysed using a polymerase chain reaction procedure. When no deletion was present in the sequence, the mRNA appeared to be polyadenylated at two major polyadenylation sites. A deletion upstream from the AATAAA sequence made the population of polyadenylated mRNAs very heterogenous at their 3' ends. A deletion downstream of the AATAAA sequence had no effect on the choice of the site. Alternative polyadenylation sites were used when the native polyadenylation site was deleted. These results are discussed in relation to data obtained with other polyadenylation sequences from both plants and animals.

A dissection of the cauliflower mosaic virus polyadenylation signal

Mutagenesis analysis of the polyadenylation [poly(A)] signal from the cauliflower mosaic virus (CaMV), a plant pararetrovirus, revealed striking differences to known vertebrate poly(A) signals. Our results show that (1) the AATAAA sequence is necessary for efficient cleavage at the poly(A) site, although the requirement for an authentic AATAAA might be less stringent in plant than in vertebrate cells; (2) surprisingly and in contrast to the majority of vertebrate poly(A) signals, the sequences downstream of the CaMV poly(A) site do not influence processing efficiency drastically although they affect the precision of cleavage; and (3) deletion of sequences upstream of the CaMV AATAAA sequence decreased processing at the CaMV site dramatically, suggesting the presence of one or several positively acting upstream elements. An oligonucleotide consisting of CaMV upstream sequences could induce the recognition of a normally silent exogenous poly(A) signal when inserted upstream of its AATAAA motif.

Potentiation of a polyadenylylation site by a downstream protein-DNA interaction

The gypsy retroposon of Drosophila melanogaster contains a sequence that potentiates upstream polyadenylylation sites. In contrast to other sequences that influence poly(A) site use, it appears to operate at the level of the DNA template. Nuclear extracts contained protein that bound to a repeated motif in the DNA. Flies with mutations that reduced transcripts polyadenylylated in the 5' long terminal repeat of gypsy contained less DNA-binding activity than wild type. A change in the repeat motif reduced both protein binding and poly(A) site potentiation. These findings provide evidence that DNA-binding proteins can regulate polyadenylylation sites.

Sequences 5' to the polyadenylation signal mediate differential poly(A) site use in hepatitis B viruses

Most genetic elements that employ reverse transcription generate a terminally redundant genomic RNA that serves as the template for this reaction. Because the identical polyadenylation signal is present in each terminally redundant segment, synthesis of this RNA requires that this signal be ignored on the first pass of the transcription machinery, then recognized and used on the second pass. We have studied the mechanism of this differential poly(A) site use in one family of retroid elements, the hepatitis B viruses (hepadnaviruses). Our results indicate that two features are involved: the presence of a variant poly(A) signal (TATAAA) and the participation of multiple sequences 5' to this signal that act to increase the efficiency of its use. Deletion of these upstream elements abolishes proper poly(A) site use, despite the presence of the poly(A) signal and downstream GT- and T-rich motifs known to be required for polyadenylation. Sequences from the corresponding regions of retroviral genomes can restore proper processing to these hepadnaviral deletion mutants. Thus, functionally analogous upstream elements exist in other classes of retroid elements, including those employing the canonical AATAAA hexanucleotide signal.

Analysis of the signals for transcription termination by purified RNA polymerase II

Eukaryotic RNA polymerase II recognizes certain DNA sequences as effective signals for transcription termination in vitro. Previously, we have shown that such termination occurs within T-rich sequences; however, not all T runs stop the enzyme nor is the efficiency of termination correlated with the length of the T run. Here we have investigated the sequence elements that signal transcription termination by purified RNA polymerase II. We have examined terminators located within introns of the human histone H3.3 gene and the human c-myc gene. Deletion analysis of the H3.3 termination region indicates that the sequences between -6 and +24 relative to the strongest termination site are sufficient to cause transcription termination. The minimal termination signal at this site has been localized to the sequence TTTTTTTC-CCTTTTTT in the nontranscribed strand. A similar but nonidentical sequence has been defined for the c-myc termination site. Since RNA polymerase II terminates transcription only within the first run of T residues in these sequences, at least part of the termination signal lies in downstream nontranscribed DNA sequences. Restriction fragment mobility analysis indicates that the H3.3 termination region contains a bend in the DNA helix. Oligonucleotides containing the minimal termination signals also cause restriction fragments to migrate with anomalous mobility. A region of the SV40 genome containing a previously characterized bend also causes RNA polymerase II to terminate transcription. We suggest that a structural element causing a bend in the DNA helix may be part of the signal for transcription termination by purified RNA polymerase II.

Efficiency of utilization of the simian virus 40 late polyadenylation site: effects of upstream sequences

The late polyadenylation signal of simian virus 40 functions with greater efficiency than the early polyadenylation signal, in turn affecting steady-state mRNA levels. Two chloramphenicol acetyltransferase (CAT) transient expression vectors, pL-EPA and pL-LPA, that differ only in their polyadenylation signals were constructed by using the early and late polyadenylation signals, respectively. In transfections of Cos, CV-1P, or HeLa cells and subsequent Northern blot analysis of CAT-specific RNA, approximately five times more steady-state CAT mRNA was produced in transfections with pL-LPA than with pL-EPA. The basis for this difference was not related to the specific promoter used or to RNA stability. Overall, the difference in steady-state mRNA levels derived from the two plasmids appeared to be attributable to intrinsic properties of the two polyadenylation signals, resulting in distinctly different cleavage and polyadenylation efficiencies. Additionally, we found that the utilization of the late polyadenylation site was dramatically reduced by deletion of sequences between 48 and 29 nucleotides 5' of the AAUAAA hexanucleotide. This reduction of mRNA levels was shown not to be caused by altered stability of mutant precursor RNAs or mRNAs, suggesting that these upstream sequences constitute an element of the late polyadenylation signal and may cause, at least to some extent, the greater efficiency of utilization of the late polyadenylation site.

Efficiency of utilization of the simian virus 40 late polyadenylation site: effects of upstream sequences

The late polyadenylation signal of simian virus 40 functions with greater efficiency than the early polyadenylation signal, in turn affecting steady-state mRNA levels. Two chloramphenicol acetyltransferase (CAT) transient expression vectors, pL-EPA and pL-LPA, that differ only in their polyadenylation signals were constructed by using the early and late polyadenylation signals, respectively. In transfections of Cos, CV-1P, or HeLa cells and subsequent Northern blot analysis of CAT-specific RNA, approximately five times more steady-state CAT mRNA was produced in transfections with pL-LPA than with pL-EPA. The basis for this difference was not related to the specific promoter used or to RNA stability. Overall, the difference in steady-state mRNA levels derived from the two plasmids appeared to be attributable to intrinsic properties of the two polyadenylation signals, resulting in distinctly different cleavage and polyadenylation efficiencies. Additionally, we found that the utilization of the late polyadenylation site was dramatically reduced by deletion of sequences between 48 and 29 nucleotides 5' of the AAUAAA hexanucleotide. This reduction of mRNA levels was shown not to be caused by altered stability of mutant precursor RNAs or mRNAs, suggesting that these upstream sequences constitute an element of the late polyadenylation signal and may cause, at least to some extent, the greater efficiency of utilization of the late polyadenylation site.

Deletion analysis of the polyadenylation signal of a pea ribulose-1,5-bisphosphate carboxylase small-subunit gene

The polyadenylation signal of a pea gene for the small subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (rbcS) has been analyzed by deletion mutagenesis and Ti plasmid-mediated gene transfer. Sequences between 6 and 137 bases upstream from the normal polyadenylation sites in this gene (bases -6 to -137) are required for functioning of these sites. In addition, bases -111 to -235 can affect 3' end formation by altering the pattern of 3' termini seen in various transcription units. Sequences between 37 and 95 bases upstream from a cryptic polyadenylation site in this gene [A. G. Hunt, DNA 7: 329-336 (1988)] are necessary for mRNA 3' end formation at this site. At least two different parts of the 3' region of this rbcS gene can serve as a downstream element for polyadenylation at the normal poly(A) addition sites in this gene. Our studies indicate that: 1. the upstream sequences required for polyadenylation in plants are different from those defined in mammalian RNA polymerase II transcription units; 2. sequences 100 or more bases upstream and downstream from poly(A) addition sites in this gene can affect poly(A) addition site choice; and 3. there are apparently redundant downstream elements for polyadenylation in this gene.

Deletions in the SV40 late polyadenylation region downstream of the AATAAA mediate similar effects on expression in various mammalian cell lines

A series of deletions in the SV40 late polyadenylation region was assayed by transient expression in a hamster fibroblast cell line. Because of differences in expression data between our results and the published results of another laboratory using a similar set of deletions introduced into a monkey kidney cell line, we studied our deletions in cells of different tissue-types and species (1). Deletion of the SV40 late polyadenylation region to 49 nucleotides downstream of the hexanucleotide AATAAA showed a small effect on gene expression, while further truncation of the region to 6 nucleotides downstream of the AATAAA showed an 85% drop in marker enzyme activity, protein levels and steady-state message levels. Another deletion in the same region, from base pair 10 to 15 past the AATAAA, which removes the wild-type site of RNA cleavage, showed a 50% drop in marker gene expression. The effects of these mutants on gene expression were similar in all of the cell lines tested and agree with other studies that DNA downstream of the AATAAA plays a role in efficient gene expression.

High-level transient expression of influenza virus proteins from a series of SV40 late and early replacement vectors

We have constructed a collection of simian virus 40 (SV40) plasmid vectors useful for transient or constitutive expression of cDNA or genomic DNA in animal cells. Most vectors contain several unique restriction sites downstream from the SV40 late or early promoter, and are available with or without the virus-specific splicing signals. The use of these vectors for transient expression in monkey cells of X47 (H3N2) influenza hemagglutinin (HA) and matrix protein (M1) was demonstrated. Membrane-bound (HAm) as well as secreted forms of the HA glycoprotein lacking the sequence of the C-terminal anchor (HA-) have been obtained. Depending on the insert, the type of vector and the amount of transfected DNA, HA levels in COS cells [Gething and Sambrook, Nature 293 (1981) 620-625] transfected with late replacement SV40 vectors vary from 10(9) (HAm) to 10(8) (HA-) molecules per transfected cell. The maximum expression levels with early replacement vectors in COS cells are at least 50 times lower. In addition to the optimalization and the characterization of the expression of each vector-coded influenza protein, cotransfections, including vectors expressing HAm, neuraminidase (NA) and M1, were undertaken. The latter experiments did not result in a measureable amount of HAm or NA in the cell culture medium, suggesting that expression of these three structural viral proteins does not result in budding of (empty) influenza particles from the cell surface.

A functional mRNA polyadenylation signal is required for transcription termination by RNA polymerase II

Polyadenylation of pre-mRNAs requires the conserved hexanucleotide AAUAAA, as well as sequences located downstream from the poly(A) addition site. The role of these sequences in the production of functional mRNAs was studied by analyzing a series of mutants containing deletions or substitutions in the SV40 early region poly(A) site. As expected, both a previously defined GU-rich downstream element and an AAUAAA sequence were required for efficient usage of the wild-type poly(A) addition site. However, when either of these elements was deleted, greatly increased levels of SV40-specific RNA were detected in the nuclei of transfected cells. Evidence is presented that this accumulation of RNA resulted from a failure of transcription termination, leading to multiple rounds of transcription of the circular templates. We conclude that the sequences required for efficient cleavage/polyadenylation of the SV40 early pre-mRNA also constitute an important element of an RNA polymerase II termination signal. A model proposing a mechanism by which the act of pre-mRNA 3' end formation is signaled to the elongating RNA polymerase, resulting in termination, is presented.

Analysis of adenovirus type 2 L1 RNA 3'-end formation in vivo and in vitro

Downstream sequence requirements for efficient cleavage and polyadenylation at the adenovirus type 2 L1 poly(A) site were determined in vivo in 293 cells and in vitro by using RNA precursors in HeLa cell nuclear extracts. The two cleavage sites used were found to differ in sensitivity to 3'-end deletion in vivo and in vitro.

Putative polyadenylation signals in nuclear genes of higher plants: a compilation and analysis

In animal and viral pre-mRNAS, the process of polyadenylation is mediated through several cis-acting poly (A) signals present upstream and downstream from poly (A) sites. The situation regarding polyadenylation of higher plant pre-mRNAS, however, has remained obscure so far. In this paper, a search for putative poly (A) signals is made by considering the published data from 46 plant genomic DNA sequences. Certain domains in the 3' untranslated regions from nuclear genes of higher plants were compiled and occurrence of sequence motifs such as AATAAA, CAYTG, YGTGTTYY and YAYTG was scored in relation to poly (A) sites. Moreover, consensus sequences for important regions in the 3' untranslated sequences and poly (A) signals were also deduced from the data. It was inferred that sequence motifs similar to poly (A) signals exist around poly (A) sites but some of them are in entirely different spatial relationship than observed in other eukaryotes. This indicates their probable non-involvement in the process of polyadenylation in higher plants necessitating a functional analysis approach to define the plant specific poly (A) signals.

Position-dependent sequence elements downstream of AAUAAA are required for efficient rabbit beta-globin mRNA 3' end formation

We previously demonstrated that a critical 35 bp region 3' of the AAUAAA is required for rabbit beta-globin mRNA 3' end formation. Recently, we synthesized and tested sequence elements derived from this region. Here, we report that a GU-rich and a U-rich sequence element are both required for efficient rabbit beta-globin mRNA 3' end formation. The efficiency of processing is restored to the wild-type level when the two elements are placed together and is greatly diminished when only one element is present. The level of 3' end formation is also decreased when the distance between the two elements is expanded. These results demonstrate that the GU-rich and U-rich elements function synergistically to restore efficient mRNA 3' end formation and that they most likely form a single requisite sequence 3' of the AAUAAA. Furthermore, we show that the effect of the GU-rich and U-rich sequence elements is position-dependent.

Polyadenylation at a cryptic site in the pBR322 portion of pSV2-neo: prevention of its utilization by the SV40 late poly(A) signal

Transcripts originating from the SV40 late promoter of pSV2-neo or pSV2-cat contain pBR322 sequences and are polyadenylated at the SV40 late poly(A) site, resulting in an RNA of 3500 nt. If the SV40(L) poly(A) signal is destroyed, late orientation transcripts are polyadenylated at a site within pBR322 sequences, yielding in an RNA of 2500 nt. This cryptic poly(A) site is located 42-46 nucleotides downstream from an AAUAAA. Utilization of the pBR322 poly(A) signal is undetectable in late orientation transcripts from pSV2-neo or pSV2-cat, although it is located 966 nucleotides upstream from the SV40(L) poly(A) signal. The pBR322 site is not utilized when the spacing between the two poly(A) signals is varied from 209 to 1913 nucleotides. The pBR322 poly(A) site was utilized only in constructs in which all or portions of the SV40(L) poly(A) signal were deleted, such as in a construct with a 7 bp deletion into the SV40(L) AATAAA and adjacent sequences.

Sequence homologies among the three yolk polypeptide (Yp) genes in Drosophila melanogaster

To identify candidates for cis-acting sequences that regulate the sex-, stage-, and cell-specific expression of three coordinately regulated yolk polypeptide genes (Yp) in Drosophila melanogaster, we have mapped the Yp3 transcript, sequenced a 4278 bp DNA fragment containing the Yp3 gene, compared Yp3 region sequences to corresponding parts of Yp1 and Yp2, and compared the predicted amino acid sequence of YP3 to YP1 and YP2. The results showed that the Yps are largely homologous in translated regions, especially in the 3' half of the genes. Untranscribed flanking regions had little homology. A conserved inverted repeat (the H-box) has homology both to vertebrate steroid hormone receptor binding sites and to the ecdysone control region of Drosophila's hsp23. These results identify sequences to mutate in order to define elements that regulate Yp gene expression and govern YP polypeptide function.

Conserved 12-bp element downstream from mRNA polyadenylation sites

Sequences downstream from the AATAAA motif in a number of cellular and viral transcription units have been compared. A 12-bp conserved element was identified in approximately half of the cases studied, and a consensus sequence TTGANNNTTTTTT was derived from a comparison of 74 such sequences. This element is located immediately (5-20 bp) downstream from the poly(A)-addition site in every case where this is known, and it is suggested that this element may be involved in the cleavage/polyadenylation reaction. This proposal is consistent with published studies on deletion mutants of downstream regions.