GO_REF:0000002

Gene Ontology annotation through association of InterPro records with GO terms

PMID:11359579

The act operon controls the level and time of C-signal production for Myxococcus xanthus development.

The C-signal, the product of csgA, is the morphogen for both fruiting body assembly and myxospore differentiation, and its production is controlled by the act operon.

"The C-signal is a morphogen that controls the assembly of fruiting bodies and the differentiation of myxospores."
csgA expression is under the control of the co-transcribed act genes, which at the time were taken to be four.

"Production of this signal, which is encoded by the csgA gene, is regulated by the act operon of four genes that are co-transcribed from the same start site."
actA, separately from the other act genes, sets the maximum level of C-signal: an actA null caps CsgA protein at a quarter of the wild-type peak. This is the gene-specific evidence behind the regulation-of-signal-transduction row.

"The act A and act B genes regulate the maximum level of the C-signal, which never rises above one-quarter of the maximum wild-type level of CsgA protein in null mutants of either gene."
The actA null phenotype is asymmetric: aggregation and rippling proceed, sporulation does not. This is the gene-specific evidence behind the regulation-of-sporulation row.

"The act A and act B mutants have the same developmental phenotype: both aggregate, neither sporulates, both prolong rippling."
ActA is assigned as a response regulator from its sequence, and ActB as a sigma-54 activator.

"By sequence homology, act A encodes a response regulator, and act B encodes a sigma-54 activator protein of the NTRC class."
ActA and ActB form one signal transduction pathway that both responds to C-signal and raises CsgA production, a positive feedback loop.

"the two gene products are part of the same signal transduction pathway. That pathway responds to C-signal and also regulates the production of CsgA protein, thus creating a positive feedback loop."

PMID:11807078

act operon control of developmental gene expression in Myxococcus xanthus.

The earlier work is summarised by its own authors as showing that the act genes direct csgA transcription, which is the basis for reading the actA effect as control of C-signal production rather than a late indirect consequence.

"Earlier work has shown that transcription of the csgA gene, which encodes the C-signal, is directed by four genes of the act operon."
act mutations leave the early, C-signal-independent developmental program intact, so the actA defect is specific to C-signal-dependent events.

"Neither the time of first expression nor the rate of expression increase was changed in actA, actB, actC, or actD mutant strains."
actA and actB null mutations depress all four aggregation and sporulation reporters expressed from 6 h of development, in contrast to the timing-only effects of actC and actD.

"The actA and actB null mutations substantially decreased the expression of all these reporters."

PMID:17189369

Mutations of the act promoter in Myxococcus xanthus.

The work was done in DK1622, the standard M. xanthus laboratory strain and reference genome.

"integrated by homologous recombination into the act region of the chromosome of DK1622, the strain which provided a standard genetic background for this work"
The act-operon sequence submitted as AF350253, from which UniProt entry Q93P59 derives, is the same sequence as in the DK1622 genome CP000113; actA is MXAN_3213.

"The entire sequence was verified in the complete sequence of the M. xanthus genome, GenBank accession number CP000113"
The operon contains five genes, actA to actE, not four as originally reported.

"All act genes, actA to actE , are expressed together and constitute an operon"
ActA is a response regulator functioning together with ActB.

"actA encodes a response regulator that works with actB"
Deleting actA depresses transcription of the act operon as strongly as deleting actB, so ActA feeds back on its own operon's expression.

"act reporter expression is substantially and equally depressed in the"
ActA and ActB are proposed to form the signal transduction pathway that receives C-signal, with ActA acting upstream of ActB.

"ActA and ActB appear to be part of a signal transduction pathway that receives input from C signal on the transmitting cell."
The proposed order of events places an unidentified C-signal receptor upstream of ActA and ActB downstream of it.

"would signal ActA, which, in turn, would activate ActB, presumably to ActB"
The final level of csgA expression is attributed specifically to actA and actB, as distinct from the timing genes actC and actD.

"The final level of csgA expression is set by actA and actB"

PMID:26369581

Two-Component Signal Transduction Systems That Regulate the Temporal and Spatial Expression of Myxococcus xanthus Sporulation Genes.

ActA is a response regulator with a GGDEF-type output domain, independently stated outside the primary papers.

"ActA is a putative response regulator that is predicted to have a GGDEF-type effector domain, which is found in diguanylate cyclase enzymes."
The actA requirement is specifically for sporulation within nascent fruiting bodies and not for aggregation, confirming the asymmetry of the 2001 phenotype.

"Mutational analyses indicate that actA and actB are required for sporulation inside nascent fruiting bodies but not for the formation of aggregates of cells"
Beyond that phenotype, actA is essentially uncharacterised, which is why this review proposes no molecular-function annotation beyond the response-regulator activity inferred from the domain.

"Little else is known about the function of actA"
The proposed direct regulation of csgA by ActB is unlikely, so the act-to-csgA effect runs through at least one unidentified intermediate. This is the reason no gene-expression regulation term is proposed for ActA.

"it seems unlikely that ActB directly regulates developmental expression of csgA"
No histidine kinase upstream of the act pathway has been identified, which is the basis of the phosphorelay knowledge gap recorded in this review.

"The histidine kinase sensor that activates ActB via phosphorylation and presumably triggers ActB-mediated expression of sporulation genes in nascent fruiting bodies has yet to be identified."

file:MYXXA/actA/actA-uniprot.txt

UniProtKB unreviewed entry Q93P59 (Q93P59_MYXXA, Response regulator ActA, Myxococcus xanthus)

The entry derives from the act-operon submission EMBL AF350253 and records the source strain as DK1622, the same standard strain used in the annotated experiments.

"STRAIN=DK1622"
The response-regulator receiver domain is matched over residues 41-157 by PROSITE PS50110.

"Response regulatory"
A phospho-accepting aspartate is predicted at residue 90, as expected for a functional receiver domain.

"4-aspartylphosphate"
The entry carries the InterPro signal-transduction response-regulator receiver signature IPR001789 that underlies the GO:0000160 annotation.

"Sig_transdc_resp-reg_receiver"