ABSTRACT The expression of sll1689 , an open reading frame from the cyanobacterium Synechocystis sp. strain PCC 6803 putatively encoding a member of the ς 70 family of sigma factors, appears to be regulated by the nitrogen control transcription factor NtcA. Disruption of sll1689 had no noticeable effect on exponential growth, identifying its product as a member of the group 2, nonessential class of ς 70 -like sigma factors; however, this disruption decreased the viability of the cells after long periods of nitrogen starvation. We have named this gene rpoD2 - V . The expression of glnN , encoding a type III glutamine synthetase, was impaired in strains bearing an inactivated copy of the rpoD2 - V gene.