Structure of transcription factor HetR required for heterocyst differentiation in cyanobacteria

Abstract

HetR is an essential regulator of heterocyst development in cyanobacteria. HetR binds to a DNA palindrome upstream of the hetP gene. We report the crystal structure of HetR from Fischerella at 3.0 Å. The protein is a dimer comprised of a central DNA-binding unit containing the N-terminal regions of the two subunits organized with two helix-turn-helix motifs; two globular flaps extending in opposite directions; and a hood over the central core formed from the C-terminal subdomains. The flaps and hood have no structural precedent in the protein database, therefore representing new folds. The structural assignments are supported by site-directed mutagenesis and DNA-binding studies. We suggest that HetR serves as a scaffold for assembly of transcription components critical for heterocyst development.

Structures of complexes comprised of Fischerella transcription factor HetR with Anabaena DNA targets

“…into the Fischerella structure and confirmed the outlines of the proposed complex by mutagenesis of three key residues in the HTH domain (14). We also modeled the peptide RGSGR and proposed three locations in HetR as possible binding sites, two of which would directly interfere with the formation of the HetR-DNA complex (14).…”

“…It has been proposed that in Anabaena, HetR recognizes a 17-bp semipalindromic DNA sequence (12); however, a shorter (15 bp) consensus sequence was reported recently (11), and longer binding sites were proposed based on the crystal structure of the HetR dimer (14). Based on the comparison of HetR binding sites within the hetP promoter of several filamentous cyanobacteria, a consensus sequence recognized by HetR can be derived (5′-tnantngnGGGtcaanCCCanca-3′; Fig.…”

“…We also modeled the peptide RGSGR and proposed three locations in HetR as possible binding sites, two of which would directly interfere with the formation of the HetR-DNA complex (14). Here we present the results of further work on the structure of the complex of the Fischerella HetR dimer with several target DNA palindromes based on Anabaena and Fischerella genome sequences upstream of the conjugate hetP genes.…”

“…We previously cloned and purified HetR from five cyanobacteria, including the thermophile Fischerella, and showed that all of them complement a hetR deletion mutant of Anabaena. We then solved the 3D structure of the Fischerella HetR dimer by X-ray crystallography (14). That structure contains four distinct domains: an extended DNA-binding unit containing helix-turnhelix (HTH) motifs comprised of the two N-terminal sequences; two histidine-rich flaps protruding on either side of the extended structure; and finally a hood comprised of the two C-terminal sequences.…”

Restricted cellular differentiation in cyanobacterial filaments

“…2-Oxoglutarate is a positive effector of NtcA, a transcription factor that triggers the expression of genes encoding proteins for the assimilation of sources of nitrogen alternative to ammonia, the preferred nitrogen source (2, 7). To specifically promote heterocyst differentiation, another regulator, HetR, is required that appears to be also a transcription factor (8). The induction of ntcA and hetR after nitrogen deprivation is mutually dependent producing an amplification loop of gene expression (9).…”