Identification of an activator protein required for the induction of fruA, a gene essential for fruiting body development in Myxococcus xanthus

Identification of a protein Ser/Thr kinase cascade that regulates essential transcriptional activators inMyxococcus xanthusdevelopment

Pkn8 is a membrane-associated protein Ser/Thr kinase (PSTK) of Myxoccocus xanthus that was previously found to associate with a novel cytoplasmic kinase, Pkn14. In the present study, MrpC, an essential transcription factor for fruA expression during fruiting body development, was identified using a genomic yeast two-hybrid screen with Pkn14 as bait. Our biochemical studies demonstrated that purified Pkn8 and Pkn14 are active kinases and that Pkn8 is able to phosphorylate Pkn14 that forms a tetramer via its C-terminal 41 residues. Moreover, Pkn14 phosphorylated purified MrpC, indicating that Pkn8 is a Pkn14 kinase and Pkn14 is an MrpC kinase. The pkn8 and pkn14 deletion strains (Deltapkn8 and Deltapkn14) developed into fruiting bodies significantly faster than that of the parent strain, DZF1. While mrpC expression was at a low level in DZF1 during vegetative growth, it was highly elevated in Deltapkn8 and Deltapkn14 during vegetative growth and development. Furthermore, FruA, usually induced at 6 h of development, was instead detected at the early stationary phase and accumulated faster during development in Deltapkn8 and Deltapkn14. Therefore, the developmental phenotype of Deltapkn8 and Deltapkn14 seems to be due to untimely FruA production mediated by elevated levels of MrpC in Deltapkn8 and Deltapkn14 during vegetative growth. As pkn14 expression was increased at the mid- and late-log. phases in DZF1 but decreased during development, the Pkn8-Pkn14 kinase cascade appears to negatively regulate mrpC expression by phosphorylating MrpC during vegetative growth. This is the first demonstration of a functional PSTK cascade in prokaryotes. mrpC expression has been proposed to be activated by MrpA and MrpB which belong to a two-component His-Asp phosphorelay signal transduction system and that MrpC autoregulate its own expression (Sun H. and Shi W., 2001 J Bacteriol 183: 4786-4795). Therefore, M. xanthus seems to utilize both eukaryotic PSTK cascade and prokaryotic His-Asp phosphorelay system to precisely regulate mrpC expression with specific timing during development.

Identification of a gene involved in polysaccharide export as a transcription target of FruA, an essential factor for Myxococcus xanthus development

Fruiting body development in Myxococcus xanthus is a multicellular event that is coordinated by exchanging intercellular signals. FruA is a transcription factor essential for fruiting body development and is thought to play a key role in the C-signal pathway. Here we present the first identification of a gene regulated by FruA. The gene was isolated from a genomic library via in vitro selection in a DNA binding assay by using the DNA-binding domain of FruA tagged with His(8) at the C-terminal end (FruA-DBD-H(8)). The gene, named fdgA (FruA-dependent gene A), encodes a protein homologous to the outer-membrane auxiliary family protein involved in the polysaccharide export system. FruA-DBD-H(8) bound the upstream promoter region of the fdgA gene from nucleotide -89 to nucleotide -64 with respect to the transcription initiation site, which was required for the induction of fdgA expression during development. fdgA mRNA induced during development was absent in a fruA deletion strain. The deletion of fdgA resulted in defective fruiting body formation and reduced sporulation efficiency (1% that of the parent strain). Moreover, FruA was required for the developmental expression of sasA, which is also involved in the biosynthesis of the lipopolysaccharide O-antigen and is required for fruiting body development. Furthermore, the expression of both fdgA and sasA was partially dependent on the C-signal. These findings expand our understanding of the signal transduction pathway mediated by FruA during development in M. xanthus.

Modulating factors for the Pkn4 kinase cascade in regulating 6-phosphofructokinase in Myxococcus xanthus

Myxococcus xanthus, a Gram-negative developmental bacterium, contains a large number of protein Ser/Thr kinases (PSTKs). Among these PSTKs, Pkn4 has been shown to be 6-phosphofructokinase (PFK) kinase. PFK associates with the regulatory domain of Pkn4 (Pkn4RD) and is activated by Pkn4-mediated phosphorylation. The activation of PFK is required to consume glycogen accumulated during early development and is essential for efficient sporulation. Using the yeast two-hybrid screen, we identified three new factors, MkapA, MkapB and MkapC, that interact with Pkn4 and each contains well-known protein-protein interaction domains. MkapB contains eight tandem repeats of the TPR (tetratrico peptide repeat) domain and its interaction with Pkn4RD was phosphorylation-dependent. MkapB remained associated with Pkn4RD. As a result, Pkn4 did not interact with PFK and its activation was inhibited. While deletion of the pfk-pkn4 operon did not inhibit fruiting body formation, the spore yield was low. In contrast, a mkapB deletion mutant exhibited a 24 h delay in fruiting body formation, accumulated less glycogen in the stationary phase and gave rise to 3.2% spore formation as opposed to 100% attained with DZF1. In addition to Pkn4, MkapA associated with other membrane-associated PSTKs, Pkn1, Pkn2, Pkn8 and Pkn9, while MkapB associated with Pkn8 and Pkn9, and MkapC with Pkn8. These results indicate that there are complex PSTK networks in M. xanthus that share common modulating factors.

Mutational analysis of the fruA promoter region demonstrates that C-Box and 5-base-pair elements are important for expression of an essential developmental gene of Myxococcus xanthus

Myxococcus xanthus uses extracellular signals during development to regulate gene expression. C-signaling regulates the expression of many genes induced after 6 h into development. FruA is a protein that is necessary for cells to respond to C-signaling, but expression of the fruA gene does not depend on C-signaling. Yet the fruA promoter region has a C box and a 5-bp element, similar to the promoter regions of several C-signal-dependent genes, where these sequences are crucial. Here, we show that the C box and 5-bp elements are important for expression of fruA, demonstrating for the first time that these sequences play a role in the expression of a gene that does not depend on C-signaling and is required for M. xanthus development.

HthA, a putative DNA-binding protein, and HthB are important for fruiting body morphogenesis in Myxococcus xanthus

In response to starvation, Myxococcus xanthus initiates a developmental programme that results in the formation of spore-filled multicellular fruiting bodies. Fruiting body formation depends on the temporal and spatial coordination of aggregation and sporulation and involves temporally and spatially coordinated changes in gene expression. This paper reports the identification of two genes, hthA and hthB, that are important for fruiting body formation. hthA and hthB are co-transcribed, and transcription of the two genes decreases strongly during development. Loss of HthA and HthB function results in delayed aggregation, a reduction in the level of sporulation, and abnormal developmental gene expression. Extracellular complementation experiments showed that the developmental defects caused by loss of HthA and HthB function are not due to the inability to synthesize an intercellular signal required for fruiting body formation. HthA, independent of HthB, is required for aggregation. HthB, alone or in combination with HthA, is required for sporulation. HthA is predicted to contain a C-terminal helix-turn-helix DNA-binding domain. Intriguingly, the N-terminal part of HthA does not exhibit significant amino acid similarity to proteins in the databases. The HthB protein lacks homologues in the databases. The results suggest that HthA is a novel DNA-binding protein, which regulates transcription of genes important for aggregation, and that HthB, alone or in combination with HthA, stimulates sporulation.

Mutational analysis of the Myxococcus xanthus Omicron4499 promoter region reveals shared and unique properties in comparison with other C-signal-dependent promoters

The bacterium Myxococcus xanthus undergoes multicellular development during times of nutritional stress and uses extracellular signals to coordinate cell behavior. C-signal affects gene expression late in development, including that of Omega4499, an operon identified by insertion of Tn5 lac into the M. xanthus chromosome. The Omega4499 promoter region has several sequences in common with those found previously to be important for expression of other C-signal-dependent promoters. To determine if these sequences are important for Omega4499 promoter activity, the effects of mutations on expression of a downstream reporter gene were tested in M. xanthus. Although the promoter resembles those recognized by Escherichia coli sigma(54), mutational analysis implied that a sigma(70)-type sigma factor likely recognizes the promoter. A 7-bp sequence known as a C box and a 5-bp element located 6 bp upstream of the C box have been shown to be important for expression of other C-signal-dependent promoters. The Omega4499 promoter region has C boxes centered at -33 and -55 bp, with 5-bp elements located 7 and 8 bp upstream, respectively. A multiple-base-pair mutation in any of these sequences reduced Omega4499 promoter activity more than twofold. Single base-pair mutations in the C box centered at -33 bp yielded a different pattern of effects on expression than similar mutations in other C boxes, indicating that each functions somewhat differently. An element from about -81 to -77 bp exerted a twofold positive effect on expression but did not appear to be responsible for the C-signal dependence of the Omega4499 promoter. Mutations in sigD and sigE, which are genes that encode sigma factors, reduced expression from the Omega4499 promoter. The results provide further insight into the regulation of C-signal-dependent genes, demonstrating both shared and unique properties among the promoter regions so far examined.

Mutational analysis of the Myxococcus xanthus Omega4400 promoter region provides insight into developmental gene regulation by C signaling

Myxococcus xanthus utilizes extracellular signals during development to coordinate cell movement, differentiation, and changes in gene expression. One of these signals, the C signal, regulates the expression of many genes, including Omega4400, a gene identified by an insertion of Tn5 lac into the chromosome. Expression of Tn5 lac Omega4400 is reduced in csgA mutant cells, which fail to perform C signaling, and the promoter region has several sequences similar to sequences found in the regulatory regions of other C-signal-dependent genes. One such gene, Omega4403, depends absolutely on the C signal for expression, and its promoter region has been characterized previously by mutational analysis. To determine if the similar sequences within the Omega4400 and Omega4403 regulatory regions function in the same way, deletion analysis and site-directed mutagenesis of the Omega4400 promoter region were performed. A 7-bp sequence centered at -49 bp, termed a C box, is identical in the Omega4400 and Omega4403 promoter regions, yet mutations in the individual base pairs affected expression from the two promoters very differently. Also, a single-base-pair change within a similar 5-bp element, which is centered at -61 bp in both promoter regions, had very different effects on the activities of the two promoters. Further mutational analysis showed that two regions are important for Omega4400 expression; one region, from -63 to -31 bp, is required for Omega4400 expression, and the other, from -86 to -81 bp, exerts a two- to fourfold effect on expression and is at least partially responsible for the C signal dependence of the Omega4400 promoter. Mutations in sigD and sigE, which are genes that encode sigma factors, abolished and reduced Omega4400 expression, respectively. Expression of Omega4400 in actB or actC mutants correlated well with the altered levels of C signal produced in these mutants. Our results provide the first detailed analysis of an M. xanthus regulatory region that depends partially on C signaling for expression and indicate that similar DNA sequences in the Omega4400 and Omega4403 promoter regions function differently.