Affiliation:
1. Department of Microbiology and Molecular Genetics, University of California, Davis, California, USA
Abstract
ABSTRACT
In cyanobacterial
Nostoc
species, substratum-dependent gliding motility is confined to specialized nongrowing filaments called hormogonia, which differentiate from vegetative filaments as part of a conditional life cycle and function as dispersal units. Here we confirm that
Nostoc punctiforme
hormogonia are positively phototactic to white light over a wide range of intensities.
N. punctiforme
contains two gene clusters (clusters 2 and 2i), each of which encodes modular cyanobacteriochrome–methyl-accepting chemotaxis proteins (MCPs) and other proteins that putatively constitute a basic chemotaxis-like signal transduction complex. Transcriptional analysis established that all genes in clusters 2 and 2i, plus two additional clusters (clusters 1 and 3) with genes encoding MCPs lacking cyanobacteriochrome sensory domains, are upregulated during the differentiation of hormogonia. Mutational analysis determined that only genes in cluster 2i are essential for positive phototaxis in
N. punctiforme
hormogonia; here these genes are designated
ptx
(for
p
hoto
t
a
x
is) genes. The cluster is unusual in containing complete or partial duplicates of genes encoding proteins homologous to the well-described chemotaxis elements CheY, CheW, MCP, and CheA. The cyanobacteriochrome-MCP gene (
ptxD
) lacks transmembrane domains and has 7 potential binding sites for bilins. The transcriptional start site of the
ptx
genes does not resemble a sigma 70 consensus recognition sequence; moreover, it is upstream of two genes encoding gas vesicle proteins (
gvpA
and
gvpC
), which also are expressed only in the hormogonium filaments of
N. punctiforme
.
Publisher
American Society for Microbiology
Subject
Molecular Biology,Microbiology
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