Identification and characterization of hetA, a gene that acts early in the process of morphological differentiation of heterocysts

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Abstract
Envelope polysaccharide is a major early diagnostic of differentiating heterocysts. The mutation in mutant EF116 of Anabaena sp. strain PCC 7120 reduces the cohesiveness of this polysaccharide. A 3.5-kilobase fragment of DNA cloned from the wild type of this Anabaena sp. was previously shown to complement this mutation. We present the nucleotide sequence of a 2,555-base-pair portion of this fragment containing an open reading frame (ORF) of 601 amino acids. Complementation analysis using deletion derivatives of the 3.5-kilobase fragment showed that the gene mutated in EF116, which we designate hetA, lies within this ORF. Transcription of hetA was induced as a result of deprivation for nitrate and yielded a monocistronic mRNA that was present at greatest abundance 7 h after nitrogen stepdown. At that time, proheterocysts could not be distinguished by light microscopy; transcription of nifHD, structural genes of nitrogenase, peaked much later. Situated 3' to hetA are 4 identical repeats of the sequence 5'-TTCAAAA-3' and 12 repeats (10 identical) of the sequence 5'-CCCCAAT-3'. The 12 repeats, present within and near the 5' end of a second ORF, are almost identical to repeats that have been reported to be present in the region between the petC and petA genes of a related cyanobacterium.