Abstract
Nitrogen-deprived filaments of wild-type or hetC Anabaena sp. produce respectively, at semiregular intervals, heterocysts and weakly fluorescent cells. Unlike heterocysts, the latter cells can divide and elongate, producing a pattern of spaced series of small cells. Because a hetR :: gfp fusion is expressed most strongly in the small cells, we propose that these small cells represent a very early stage of heterocyst differentiation. hetC :: gfp is expressed most strongly in proheterocysts and heterocysts.