Abstract

The C-signal is a morphogen that controls the assembly of fruiting bodies and the differentiation of myxospores. 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. 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 act A and act B mutants have the same developmental phenotype: both aggregate, neither sporulates, both prolong rippling. By sequence homology, act A encodes a response regulator, and act B encodes a sigma-54 activator protein of the NTRC class. The similar phenotypes of act A and act B deletion mutants suggest that 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. The act C and act D genes regulate the time pattern of CsgA production, while achieving the same maximum level. An act C null mutant raises CsgA production 15 h earlier than the wild type, whereas an act D null mutant does so 6 h later than wild type. The loop explains how the C-signal rises continuously from early development to a peak at the time of sporulation, and the act genes govern the time course of that rise.

Ancillary

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Publication History

Issue online: 21 December 2001

Version of record online: 21 December 2001

Accepted 2 March, 2001.

Supporting Information

Fig. 1. Sequence alignments of a conceptual translation of ActA with E. coli cheY (Stock et al., 1985) are shown in the upper three paragraphs of sequence. The lower five paragraphs show ActA starting with it's residue 61 and extending to its C-terminus. It is shown aligned with part of pleD of Caulobacter crescentus and part of celR2 of Rhizobium leguminosarum (Sommer and Newton, 1989; Aldridge and Jenal, 1999; Ausmees et al., 1999). Identical amino acid residues are highlighted in black, and similar residues are highlighted in grey.

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