Double molecular mimicry in Escherichia coli: binding of ribosomal protein L20 to its two sites in mRNA is similar to its binding to 23S rRNA

Extraribosomal Functions of Bacterial Ribosomal Proteins-An Update, 2023

Ribosomal proteins (r-proteins) are abundant, highly conserved, and multifaceted cellular proteins in all domains of life. Most r-proteins have RNA-binding properties and can form protein-protein contacts. Bacterial r-proteins govern the co-transcriptional rRNA folding during ribosome assembly and participate in the formation of the ribosome functional sites, such as the mRNA-binding site, tRNA-binding sites, the peptidyl transferase center, and the protein exit tunnel. In addition to their primary role in a cell as integral components of the protein synthesis machinery, many r-proteins can function beyond the ribosome (the phenomenon known as moonlighting), acting either as individual regulatory proteins or in complexes with various cellular components. The extraribosomal activities of r-proteins have been studied over the decades. In the past decade, our understanding of r-protein functions has advanced significantly due to intensive studies on ribosomes and gene expression mechanisms not only in model bacteria like Escherichia coli or Bacillus subtilis but also in little-explored bacterial species from various phyla. The aim of this review is to update information on the multiple functions of r-proteins in bacteria.

Discovery of 20 novel ribosomal leader candidates in bacteria and archaea

BACKGROUND

RNAs perform many functions in addition to supplying coding templates, such as binding proteins. RNA-protein interactions are important in multiple processes in all domains of life, and the discovery of additional protein-binding RNAs expands the scope for studying such interactions. To find such RNAs, we exploited a form of ribosomal regulation. Ribosome biosynthesis must be tightly regulated to ensure that concentrations of rRNAs and ribosomal proteins (r-proteins) match. One regulatory mechanism is a ribosomal leader (r-leader), which is a domain in the 5' UTR of an mRNA whose genes encode r-proteins. When the concentration of one of these r-proteins is high, the protein binds the r-leader in its own mRNA, reducing gene expression and thus protein concentrations. To date, 35 types of r-leaders have been validated or predicted.

RESULTS

By analyzing additional conserved RNA structures on a multi-genome scale, we identified 20 novel r-leader structures. Surprisingly, these included new r-leaders in the highly studied organisms Escherichia coli and Bacillus subtilis. Our results reveal several cases where multiple unrelated RNA structures likely bind the same r-protein ligand, and uncover previously unknown r-protein ligands. Each r-leader consistently occurs upstream of r-protein genes, suggesting a regulatory function. That the predicted r-leaders function as RNAs is supported by evolutionary correlations in the nucleotide sequences that are characteristic of a conserved RNA secondary structure. The r-leader predictions are also consistent with the locations of experimentally determined transcription start sites.

CONCLUSIONS

This work increases the number of known or predicted r-leader structures by more than 50%, providing additional opportunities to study structural and evolutionary aspects of RNA-protein interactions. These results provide a starting point for detailed experimental studies.

Overexpressed L20 Rescues 50S Ribosomal Subunit Assembly Defects of bipA-Deletion in Escherichia coli

The BipA (BPI-inducible protein A) protein is highly conserved in a large variety of bacteria and belongs to the translational GTPases, based on sequential and structural similarities. Despite its conservation in bacteria, bipA is not essential for cell growth under normal growth conditions. However, at 20°C, deletion of bipA causes not only severe growth defects but also several phenotypic changes such as capsule production, motility, and ribosome assembly, indicating that it has global regulatory properties. Our recent studies revealed that BipA is a novel ribosome-associating GTPase, whose expression is cold-shock-inducible and involved in the incorporation of the ribosomal protein (r-protein) L6. However, the precise mechanism of BipA in 50S ribosomal subunit assembly is not completely understood. In this study, to demonstrate the role of BipA in the 50S ribosomal subunit and possibly to find an interplaying partner(s), a genomic library was constructed and suppressor screening was conducted. Through screening, we found a suppressor gene, rplT, encoding r-protein L20, which is assembled at the early stage of ribosome assembly and negatively regulates its own expression at the translational level. We demonstrated that the exogenous expression of rplT restored the growth of bipA-deleted strain at low temperature by partially recovering the defects in ribosomal RNA processing and ribosome assembly. Our findings suggest that the function of BipA is pivotal for 50S ribosomal subunit biogenesis at a low temperature and imply that BipA and L20 may exert coordinated actions for proper ribosome assembly under cold-shock conditions.

rRNA Mimicry in RNA Regulation of Gene Expression

The rRNA is the largest and most abundant RNA in bacterial and archaeal cells. It is also one of the best-characterized RNAs in terms of its structural motifs and sequence variation. Production of ribosome components including >50 ribosomal proteins (r-proteins) consumes significant cellular resources. Thus, RNA cis-regulatory structures that interact with r-proteins to repress further r-protein synthesis play an important role in maintaining appropriate stoichiometry between r-proteins and rRNA. Classically, such mRNA structures were thought to directly mimic the rRNA. However, more than 30 years of research has demonstrated that a variety of different recognition and regulatory paradigms are present. This review will demonstrate how structural mimicry between the rRNA and mRNA cis-regulatory structures may take many different forms. The collection of mRNA structures that interact with r-proteins to regulate r-protein operons are best characterized in Escherichia coli, but are increasingly found within species from nearly all phyla of bacteria and several archaea. Furthermore, they represent a unique opportunity to assess the plasticity of RNA structure in the context of RNA-protein interactions. The binding determinants imposed by r-proteins to allow regulation can be fulfilled in many ways. Some r-protein-interacting mRNAs are immediately obvious as rRNA mimics from primary sequence similarity, others are identifiable only after secondary or tertiary structure determination, and some show no obvious similarity. In addition, across different bacterial species a host of different mechanisms of action have been characterized, showing that there is no simple one-size-fits-all solution.

Pervasive Regulatory Functions of mRNA Structure Revealed by High-Resolution SHAPE Probing

mRNAs can fold into complex structures that regulate gene expression. Resolving such structures de novo has remained challenging and has limited our understanding of the prevalence and functions of mRNA structure. We use SHAPE-MaP experiments in living E. coli cells to derive quantitative, nucleotide-resolution structure models for 194 endogenous transcripts encompassing approximately 400 genes. Individual mRNAs have exceptionally diverse architectures, and most contain well-defined structures. Active translation destabilizes mRNA structure in cells. Nevertheless, mRNA structure remains similar between in-cell and cell-free environments, indicating broad potential for structure-mediated gene regulation. We find that the translation efficiency of endogenous genes is regulated by unfolding kinetics of structures overlapping the ribosome binding site. We discover conserved structured elements in 35% of UTRs, several of which we validate as novel protein binding motifs. RNA structure regulates every gene studied here in a meaningful way, implying that most functional structures remain to be discovered.

The role of mRNA structure in bacterial translational regulation

The characteristics of bacterial messenger RNAs (mRNAs) that influence translation efficiency provide many convenient handles for regulation of gene expression, especially when coupled with the processes of transcription termination and mRNA degradation. An mRNA's structure, especially near the site of initiation, has profound consequences for how readily it is translated. This property allows bacterial gene expression to be altered by changes to mRNA structure induced by temperature, or interactions with a wide variety of cellular components including small molecules, other RNAs (such as sRNAs and tRNAs), and RNA-binding proteins. This review discusses the links between mRNA structure and translation efficiency, and how mRNA structure is manipulated by conditions and signals within the cell to regulate gene expression. The range of RNA regulators discussed follows a continuum from very complex tertiary structures such as riboswitch aptamers and ribosomal protein-binding sites to thermosensors and mRNA:sRNA interactions that involve only base-pairing interactions. Furthermore, the high degrees of diversity observed for both mRNA structures and the mechanisms by which inhibition of translation occur have significant consequences for understanding the evolution of bacterial translational regulation. WIREs RNA 2017, 8:e1370. doi: 10.1002/wrna.1370 For further resources related to this article, please visit the WIREs website.

Co-evolution of Bacterial Ribosomal Protein S15 with Diverse mRNA Regulatory Structures

RNA-protein interactions are critical in many biological processes, yet how such interactions affect the evolution of both partners is still unknown. RNA and protein structures are impacted very differently by mechanisms of genomic change. While most protein families are identifiable at the nucleotide level across large phylogenetic distances, RNA families display far less nucleotide similarity and are often only shared by closely related bacterial species. Ribosomal protein S15 has two RNA binding functions. First, it is a ribosomal protein responsible for organizing the rRNA during ribosome assembly. Second, in many bacterial species S15 also interacts with a structured portion of its own transcript to negatively regulate gene expression. While the first interaction is conserved in most bacteria, the second is not. Four distinct mRNA structures interact with S15 to enable regulation, each of which appears to be independently derived in different groups of bacteria. With the goal of understanding how protein-binding specificity may influence the evolution of such RNA regulatory structures, we examine whether examples of these mRNA structures are able to interact with, and regulate in response to, S15 homologs from organisms containing distinct mRNA structures. We find that despite their shared RNA binding function in the rRNA, S15 homologs have distinct RNA recognition profiles. We present a model to explain the specificity patterns observed, and support this model by with further mutagenesis. After analyzing the patterns of conservation for the S15 protein coding sequences, we also identified amino acid changes that alter the binding specificity of an S15 homolog. In this work we demonstrate that homologous RNA-binding proteins have different specificity profiles, and minor changes to amino acid sequences, or to RNA structural motifs, can have large impacts on RNA-protein recognition.

Regulation of the rplY gene encoding 5S rRNA binding protein L25 in Escherichia coli and related bacteria

Ribosomal protein (r-protein) L25 is one of the three r-proteins (L25, L5, L18) that interact with 5S rRNA in eubacteria. Specific binding of L25 with a certain domain of 5S r-RNA, a so-called loop E, has been studied in detail, but information about regulation of L25 synthesis has remained totally lacking. In contrast to the rplE (L5) and rplR (L18) genes that belong to the polycistronic spc-operon and are regulated at the translation level by r-protein S8, the rplY (L25) gene forms an independent transcription unit. The main goal of this work was to study the regulation of the rplY expression in vivo. We show that the rplY promoter is down-regulated by ppGpp and its cofactor DksA in response to amino acid starvation. At the level of translation, the rplY expression is subjected to the negative feedback control. The 5'-untranslated region of the rplY mRNA comprises specific sequence/structure features, including an atypical SD-like sequence, which are highly conserved in a subset of gamma-proteobacterial families. Despite the lack of a canonical SD element, the rplY'-'lacZ single-copy reporter showed unusually high translation efficiency. Expression of the rplY gene in trans decreased the translation yield, indicating the mechanism of autogenous repression. Site-directed mutagenesis of the rplY 5' UTR revealed an important role of the conserved elements in the translation control. Thus, the rplY expression regulation represents one more example of regulatory pathways that control ribosome biogenesis in Escherichia coli and related bacteria.

Multiple ways to regulate translation initiation in bacteria: Mechanisms, regulatory circuits, dynamics

To adapt their metabolism rapidly and constantly in response to environmental variations, bacteria often target the translation initiation process, during which the ribosome assembles on the mRNA. Here, we review different mechanisms of regulation mediated by cis-acting elements, sRNAs and proteins, showing, when possible, their intimate connection with the translational apparatus. Indeed the ribosome itself could play a direct role in several regulatory mechanisms. Different features of the regulatory signals (sequences, structures and their positions on the mRNA) are contributing to the large variety of regulatory mechanisms. Ribosome heterogeneity, variation of individual cells responses and the spatial and temporal organization of the translation process add more layers of complexity. This hampers to define manageable set of rules for bacterial translation initiation control.

Most RNAs regulating ribosomal protein biosynthesis in Escherichia coli are narrowly distributed to Gammaproteobacteria

In Escherichia coli, 12 distinct RNA structures within the transcripts encoding ribosomal proteins interact with specific ribosomal proteins to allow autogenous regulation of expression from large multi-gene operons, thus coordinating ribosomal protein biosynthesis across multiple operons. However, these RNA structures are typically not represented in the RNA Families Database or annotated in genomic sequences databases, and their phylogenetic distribution is largely unknown. To investigate the extent to which these RNA structures are conserved across eubacterial phyla, we created multiple sequence alignments representing 10 of these messenger RNA (mRNA) structures in E. coli. We find that while three RNA structures are widely distributed across many phyla of bacteria, seven of the RNAs are narrowly distributed to a few orders of Gammaproteobacteria. To experimentally validate our computational predictions, we biochemically confirmed dual L1-binding sites identified in many Firmicute species. This work reveals that RNA-based regulation of ribosomal protein biosynthesis is used in nearly all eubacterial phyla, but the specific RNA structures that regulate ribosomal protein biosynthesis in E. coli are narrowly distributed. These results highlight the limits of our knowledge regarding ribosomal protein biosynthesis regulation outside of E. coli, and the potential for alternative RNA structures responsible for regulating ribosomal proteins in other eubacteria.

Probing ribosomal protein-RNA interactions with an external force

Ribosomal (r-) RNA adopts a well-defined structure within the ribosome, but the role of r-proteins in stabilizing this structure is poorly understood. To address this issue, we use optical tweezers to unfold RNA fragments in the presence or absence of r-proteins. Here, we focus on Escherichia coli r-protein L20, whose globular C-terminal domain (L20C) recognizes an irregular stem in domain II of 23S rRNA. L20C also binds its own mRNA and represses its translation; binding occurs at two different sites--i.e., a pseudoknot and an irregular stem. We find that L20C makes rRNA and mRNA fragments encompassing its binding sites more resistant to mechanical unfolding. The regions of increased resistance correspond within two base pairs to the binding sites identified by conventional methods. While stabilizing specific RNA structures, L20C does not accelerate their formation from alternate conformations--i.e., it acts as a clamp but not as a chaperone. In the ribosome, L20C contacts only one side of its target stem but interacts with both strands, explaining its clamping effect. Other r-proteins bind rRNA similarly, suggesting that several rRNA structures are stabilized by "one-side" clamping.

Regulation of translation initiation by RNA binding proteins

RNA binding proteins are capable of regulating translation initiation by a variety of mechanisms. Although the vast majority of these regulatory mechanisms involve translational repression, one example of translational activation has been characterized in detail. The RNA recognition targets of these regulatory proteins exhibit a wide range in structural complexity, with some proteins recognizing complex pseudoknot structures and others binding to simple RNA hairpins and/or short repeated single-stranded sequences. In some instances the bound protein directly competes with ribosome binding, and in other instances the bound protein promotes formation of an RNA structure that inhibits ribosome binding. Examples also exist in which the bound protein traps the ribosome in a complex that is incapable of initiating translation.

SrmB, a DEAD-box helicase involved in Escherichia coli ribosome assembly, is specifically targeted to 23S rRNA in vivo

DEAD-box proteins play specific roles in remodeling RNA or ribonucleoprotein complexes. Yet, in vitro, they generally behave as nonspecific RNA-dependent ATPases, raising the question of what determines their specificity in vivo. SrmB, one of the five Escherichia coli DEAD-box proteins, participates in the assembly of the large ribosomal subunit. Moreover, when overexpressed, it compensates for a mutation in L24, the ribosomal protein (r-protein) thought to initiate assembly. Here, using the tandem affinity purification (TAP) procedure, we show that SrmB forms a complex with r-proteins L4, L24 and a region near the 5'-end of 23S rRNA that binds these proteins. In vitro reconstitution experiments show that the stability of this complex reflects cooperative interactions of SrmB with L4, L24 and rRNA. These observations are consistent with an early role of SrmB in assembly and explain the genetic link between SrmB and L24. Besides its catalytic core, SrmB possesses a nonconserved C-terminal extension that, we show, is not essential for SrmB function and specificity. In this regard, SrmB differs from DbpA, another DEAD-box protein involved in ribosome assembly.

Repression of galP, the galactose transporter in Escherichia coli, requires the specific regulator of N-acetylglucosamine metabolism

Soupene et al. [J. Bacteriol. (2003) 185 5611-5626] made the unexpected observation that the presence of a mutation, in the gene for the N-acetylglucosamine repressor, nagC, increased the growth rate of Escherichia coli MG1655 on galactose, an unrelated sugar. We have found that NagC, binds to a single, high-affinity site overlapping the promoter of galP (galactose permease) gene and that expression of galP is repressed by a combination of NagC, GalR and GalS. In addition to the previously identified galOE operator, other gal operators further upstream are required for full repression. GalS has a specific role, as it binds with higher affinity to one of the upstream operators but its effect in vivo is only observed in the presence of GalR. Regulation of galP by three specific repressors, NagC, GalR and GalS is unusual in that it involves multiple, specific regulators from two different areas of metabolism. This novel regulation seems to be particular for E. coli and its nearest neighbour, Shigella. Other bacteria with galP orthologues, although retaining the metK-galP gene order, do not have the NagC site. Although quantitative effects were strain specific, nagC mutations increased the growth rate on galactose of all E. coli strains tested.

Ribosomal Proteins in the Spotlight

The assignment of specific ribosomal functions to individual ribosomal proteins is difficult due to the enormous cooperativity of the ribosome; however, important roles for distinct ribosomal proteins are becoming evident. Although rRNA has a major role in certain aspects of ribosomal function, such as decoding and peptidyl-transferase activity, ribosomal proteins are nevertheless essential for the assembly and optimal functioning of the ribosome. This is particularly true in the context of interactions at the entrance pore for mRNA, for the translation-factor binding site and at the tunnel exit, where both chaperones and complexes associated with protein transport through membranes bind.

A new regulatory circuit in ribosomal protein operons: S2-mediated control of the rpsB-tsf expression in vivo

Autogenous regulation is a general strategy of balancing ribosomal protein synthesis in bacteria. Control mechanisms have been studied in detail for most of ribosomal protein operons, except for rpsB-tsf encoding essential r-protein S2 and elongation factor Ts, where even the promoter has remained unknown. By using single-copy translational fusions with the chromosomal lacZ gene and Western-blot analysis, we demonstrate here that S2 serves as a negative regulator of both rpsB and tsf expression in vivo, acting at a single target within the rpsB 5'-untranslated region (5'-UTR). As determined by primer extension, transcription of the Escherichia coli rpsB-tsf operon starts 162 nucleotides upstream of the rpsB initiation codon at a single promoter TGTGGTATAAA belonging to the extended -10 promoter class. Both the promoter signature and the 5'-UTR structure of the rpsB gene appear to be highly conserved in gamma-proteobacteria. Deletion analysis of the rpsB 5'-UTR within rpsB'-'lacZ fusions has revealed that an operator region involved in the S2 autoregulation comprises conserved structural elements located upstream of the rpsB ribosome binding site. The S2-mediated autogenous control is impaired in rpsB mutants and, more surprisingly, in the rpsA mutant producing decreased amounts of truncated r-protein S1 (rpsAIS10), indicating that S2 might act as a repressor in cooperation with S1.

Mutations in residues involved in zinc binding in the catalytic site of Escherichia coli threonyl-tRNA synthetase confer a dominant lethal phenotype

Escherichia coli threonyl-tRNA synthetase is a homodimeric protein that acts as both an enzyme and a regulator of gene expression: the protein aminoacylates tRNA(Thr) isoacceptors and binds to its own mRNA, inhibiting its translation. The enzyme contains a zinc atom in its active site, which is essential for the recognition of threonine. Mutations in any of the three amino acids forming the zinc-binding site inactivate the enzyme and have a dominant negative effect on growth if the corresponding genes are placed on a multicopy plasmid. We show here that this particular property is not due to the formation of inactive heterodimers, the titration of tRNA(Thr) by an inactive enzyme, or its misaminoacylation but is, rather, due to the regulatory function of threonyl-tRNA synthetase. Overproduction of the inactive enzyme represses the expression of the wild-type chromosomal copy of the gene to an extent incompatible with bacterial growth.

Escherichia coli ribosomal protein L20 binds as a single monomer to its own mRNA bearing two potential binding sites

Ribosomal protein L20 is crucial for the assembly of the large ribosomal subunit and represses the translation of its own mRNA. L20 mRNA carries two L20-binding sites, the first folding into a pseudoknot and the second into an imperfect stem and loop. These two sites and the L20-binding site on 23S ribosomal RNA are recognized similarly using a single RNA-binding site located on one face of L20. In this work, using gel filtration and fluorescence cross-correlation spectroscopy (FCCS) experiments, we first exclude the possibility that L20 forms a dimer, which would allow each monomer to bind one site of the mRNA. Secondly we show, using affinity purification and FCCS experiments, that only one molecule of L20 binds to the L20 mRNA despite the presence of two potential binding sites. Thirdly, using RNA chemical probing, we show that the two L20-binding sites are in interaction. This interaction provides an explanation for the single occupancy of the mRNA. The two interacting sites could form a single hybrid site or the binding of L20 to a first site may inhibit binding to the second. Models of regulation compatible with our data are discussed.

Ribosomal protein L20 controls expression of the Bacillus subtilis infC operon via a transcription attenuation mechanism

In contrast to Escherichia coli no molecular mechanism controlling the biosynthesis of ribosomal proteins has been elucidated in Gram-positive organisms. Here we show that the expression of the Bacillus subtilis infC-rpmI-rplT operon encoding translation factor IF3 and the ribosomal proteins L35 and L20 is autoregulated by a complex transcription attenuation mechanism. It implicates a 200-bp leader region upstream of infC which contains two conserved regulatory elements, one of which can act as a transcription terminator. Using in vitro and in vivo approaches we show that expression of the operon is regulated at the level of transcription elongation by a change in the structure of the leader mRNA which depends upon the presence of ribosomal protein L20. L20 binds to a phylogenetically conserved domain and provokes premature transcription termination at the leader terminator. Footprint and toeprint experiments support a regulatory model involving molecular mimicry between the L20-binding sites on 23S rRNA and the mRNA. Our data suggest that Nomura's model of ribosomal protein biosynthesis based on autogenous control and molecular mimicry is also valid in Gram-positive organisms.

Coexistence of two protein folding states in the crystal structure of ribosomal protein L20

The recent finding of intrinsically unstructured proteins defies the classical structure-function paradigm. However, owing to their flexibility, intrinsically unstructured proteins generally escape detailed structural investigations. Consequently little is known about the extent of conformational disorder and its role in biological functions. Here, we present the X-ray structure of the unbound ribosomal protein L20, the long basic amino-terminal extension of which has been previously interpreted as fully disordered in the absence of RNA. This study provides the first detailed picture of two protein folding states trapped together in a crystal and indicates that unfolding occurs in discrete regions of the whole protein, corresponding mainly to RNA-binding residues. The electrostatic destabilization of the long alpha-helix and a structural communication between the two L20 domains are reminiscent of those observed in calmodulin. The detailed comparison of the two conformations observed in the crystal provides new insights into the role of unfolded extensions in ribosomal assembly.

Regulation of translation via mRNA structure in prokaryotes and eukaryotes

The mechanism of initiation of translation differs between prokaryotes and eukaryotes, and the strategies used for regulation differ accordingly. Translation in prokaryotes is usually regulated by blocking access to the initiation site. This is accomplished via base-paired structures (within the mRNA itself, or between the mRNA and a small trans-acting RNA) or via mRNA-binding proteins. Classic examples of each mechanism are described. The polycistronic structure of mRNAs is an important aspect of translational control in prokaryotes, but polycistronic mRNAs are not usable (and usually not produced) in eukaryotes. Four structural elements in eukaryotic mRNAs are important for regulating translation: (i) the m7G cap; (ii) sequences flanking the AUG start codon; (iii) the position of the AUG codon relative to the 5' end of the mRNA; and (iv) secondary structure within the mRNA leader sequence. The scanning model provides a framework for understanding these effects. The scanning mechanism also explains how small open reading frames near the 5' end of the mRNA can down-regulate translation. This constraint is sometimes abrogated by changing the structure of the mRNA, sometimes with clinical consequences. Examples are described. Some mistaken ideas about regulation of translation that have found their way into textbooks are pointed out and corrected.

The N-terminal extension of Escherichia coli ribosomal protein L20 is important for ribosome assembly, but dispensable for translational feedback control

The Escherichia coli autoregulatory ribosomal protein L20 consists of two structurally distinct domains. The C-terminal domain is globular and sits on the surface of the large ribosomal subunit whereas the N-terminal domain has an extended shape and penetrates deep into the RNA-rich core of the subunit. Many other ribosomal proteins have analogous internal or terminal extensions. However, the biological functions of these extended domains remain obscure. Here we show that the N-terminal tail of L20 is important for ribosome assembly in vivo. Indeed, a truncated version of L20 without its N-terminal tail is unable to complement the deletion of rplT, the gene encoding L20. In addition, this L20 truncation confers a lethal-dominant phenotype, suggesting that the N-terminal domain is essential for cell growth because it could be required for ribosome assembly. Supporting this hypothesis, partial deletions of the N-terminal tail of the protein are shown to cause a slow-growth phenotype due to altered ribosome assembly in vivo as large amounts of intermediate 40S ribosomal particles accumulate. In addition to being a ribosomal protein, L20 also acts as an autogenous repressor. Using L20 truncations, we also show that the N-terminal tail of L20 is dispensable for autogenous control.