hetC, a gene coding for a protein similar to bacterial ABC protein exporters, is involved in early regulation of heterocyst differentiation in Anabaena sp. strain PCC 7120

Abstract

Transposon-generated mutant C3 of Anabaena sp. strain PCC 7120 is unable to form heterocysts upon deprivation of combined nitrogen but forms a pattern of spaced, weakly fluorescent cells after 2 days of deprivation. Sequence analysis of chromosomal DNA adjacent to the ends of transposon Tn5-1058 in mutant C3 showed a 1,044-amino-acid open reading frame, designated hetC, whose predicted protein product throughout its C-terminal two-thirds has extensive similarity to the HlyB family of bacterial protein exporters. Its N-terminal third is unique and does not resemble any known protein. hetC lies 1,165 bp 5 from the previously described gene hetP. Reconstruction of the C3 mutation and its complementation in trans with a wild-type copy of hetC confirmed that hetC has an essential regulatory role early in heterocyst development. hetC is induced ca. 4 h after nitrogen stepdown, hours after induction of hetR. Expression of hetC depends on HetR and may depend on HetC. Highly similar sequences are present 5 from the initiation codons and in the 3 untranslated regions of hetC and of two heterocyst-specific genes, devA and hetP.Upon deprivation of a source of combined nitrogen, certain filamentous cyanobacteria initiate a sequence of morphological and physiological changes that result in the differentiation of specialized cells called heterocysts. These cells, which form at semiregular intervals along the filaments, provide a microaerobic environment and a supply of ATP and reductant for nitrogenase. During the course of differentiation of a vegetative cell into a heterocyst, a thick envelope is synthesized. This envelope consists of an external, polysaccharide layer and an internal, laminated layer of glycolipids that functions to limit gas permeation. To provide a route, unblocked by the glycolipid layer of the envelope, for nutrients to move between the heterocyst and an adjacent vegetative cell, a channel, or pore, is formed that contains a neck region of reduced diameter, at the end of which the plasma membranes of the heterocyst and vegetative cell are narrowly separated. Structures called microplasmodesmata appear to join the two plasma membranes. The inner, thylakoid membranes are rearranged within heterocysts, with formation of a honeycomb of membranes, rich in oxidative enzymes, near the poles of the developing heterocyst. This membrane system may function in the scavenging of oxygen that enters the heterocyst through the pore. Within the developing heterocyst, oxygen production is blocked by the inactivation of components of photosystem II; specific proteases are activated; and RuBP carboxylase, the principal CO 2 -fixing enzyme, disappears early during differentiation. Late in differentiation, several genomic rearrangements take place and nitrogenase proteins are abundantly synthesized (for recent reviews, see references 9, 51, and 53). How these processes are regulated remains unclear.We describe the transposon tagging, cloning, and analysis of a gene that appears to be involved specifically in the early regula...

Tansley Review No. 107. Heterocyst and akinete differentiation in cyanobacteria

“…The gene encoding the ABC transporter is usually linked to the gene encoding the protein that is exported, prompting the suggestion that HetC may export HetP (Wolk, 1996). Transcription of hetC requires both HetR and a functional HetC protein, implying that hetC, like hetR (Black et al, 1993), is autoregulatory (Khudyakov & Wolk, 1997). The role of hetC remains unclear, although it is essential at an early stage of differentiation, implying that it must regulate the expression of later-acting, heterocyst-specific genes.…”

“…ndez-Pin4 as et al, 1994), whereas the protein encoded by hetC has extensive similarity to members of a class of bacterial ATP-binding cassette (ABC) membrane transport proteins involved in the export of pore-forming toxic polypeptides (Khudyakov & Wolk, 1997). The gene encoding the ABC transporter is usually linked to the gene encoding the protein that is exported, prompting the suggestion that HetC may export HetP (Wolk, 1996).…”

“…Three genes, hetA, hetB (later renamed hepA and hepB, respectively ; Ernst et al, 1992) and hepC, are required for the synthesis or stabilization of the polysaccharide layer (Wolk et al, 1988 ;Holland & Wolk, 1990 ;Zhu et al, 1998). HepA resembles ATP-binding transport proteins (Khudyakov & Wolk, 1997). Its transcription is dependent on hetR (a gene essential for heterocyst development ; see section III.5.…”

“…Two further genes, hetP (Fera! ndez-Pin4 as et al, 1994) and hetC (Khudyakov & Wolk, 1997), are important for heterocyst development. Mutants deficient in each of these genes fail to produce proheterocysts, although a pattern of spaced, weakly fluorescent cells is evident 2 d following the removal of combined N, implying that differentiation is arrested at a very early stage (Khudyakov & Wolk, 1997 ;Wolk, 1996).…”

“…ndez-Pin4 as et al, 1994) and hetC (Khudyakov & Wolk, 1997), are important for heterocyst development. Mutants deficient in each of these genes fail to produce proheterocysts, although a pattern of spaced, weakly fluorescent cells is evident 2 d following the removal of combined N, implying that differentiation is arrested at a very early stage (Khudyakov & Wolk, 1997 ;Wolk, 1996). As with hetR, multiple copies of hetP result in the production of multiple heterocysts, although unlike hetR, the gene is not autoregulatory, and supernumerary copies of hetP do not induce heterocyst formation in the presence of combined N (Ferna!…”