THE HGLK GENE IS REQUIRED FOR LOCALIZATION OF HETEROCYST-SPECIFIC GLYCOLIPIDS IN THE CYANOBACTERIUM ANABAENA SP STRAIN PCC-7120

Mutant strain 543 of the cyanobacterium Anabaena sp. strain PCC 7120 w as originally isolated as a Fox mutant following chemical mutagenesis. Ultrastructural analysis shows that in nitrogen-replete media the veg etative cells of the mutant are more cylindrical acid have thicker sep ta than those of the wild type, while in nitrogen-free media the mutan t heterocysts lack the normal glycolipid layer external to the cell wa ll. Although this layer is absent, strain 543 heterocysts nevertheless contain heterocyst-specific glycolipids, as determined by thin-layer chromatography. The mutation in strain 543 is in a gene we have named hglK, encoding a protein of 727 amino acids. The wild-type HglK protei n appears to contain four membrane-spanning regions followed by 36 rep eats of a degenerate pentapeptide sequence, AXLXX. The mutation in str ain 543 introduces a termination codon immediately upstream of the pen tapeptide repeat region. A mutant constructed by insertion of an antib iotic resistance cassette near the beginning of the hglK gene has the same phenotype as strain 543. We propose that hglK encodes a protein n ecessary for the localization of heterocyst glycolipids and that this function requires the pentapeptide repeats of the HglK protein.