A specific role of the cyanobacterial PipX factor in the heterocysts of Anabaena sp. strain PCC 7120

VALLADARES A; RODRÍGUEZ V; CAMARGO S; MARTÍNEZ NOËL G; HERRERO A; LUQUE I

JOURNAL OF BACTERIOLOGY

AMER SOC MICROBIOLOGY

Año: 2011 vol. 193 p. 1172 - 1182

0021-9193

The PipX factor is a regulatory protein that seems to occur only in cyanobacteria. In the filamentous, heterocyst-forming Anabaena sp. strain PCC 7120, ORF asr0485, identified as the pipX gene, is mainly expressed under conditions of combined-nitrogen deprivation dependent on the global N regulator NtcA and the heterocyst-specific regulator HetR. Primer extension and RACE analyses detected three transcription start points corresponding to a canonical NtcA-activated promoter (to which direct binding of NtcA was observed), an NtcA- and HetR-dependent promoter and a consensus-type promoter, the latter with putative -35 and -10 determinants. Activation of pipX took place in cells differentiating into heterocysts at intermediate to late stages of the process. Accordingly, disruption of pipX led to impaired diazotrophic growth, reduced nitrogenase activity and impaired activation of the nitrogenase structural genes. The nitrogenase activity of the mutant was low under oxic conditions likely resulting from inefficient protection against oxygen. In line with this, the activation of coxB2A2C2 and coxB3A3C3 operons encoding heterocyst-specific terminal respiratory oxidases responsible for internal oxygen removal was deficient in the pipX mutant. Therefore, the Anabaena PipX factor shows a spatiotemporal specificity contributing to normal heterocyst function including full activation of the nitrogenase structural genes and genes of the nitrogenase protective features of the heterocyst.