ROLE OF ALTERNATIVE A FACTOR ALGU IN ENCYSTMENT OF AZOTOBACTER-VINELANDII

Alginate is essential for encystment in Azotobacter vinelandii, Transc ription of the algD gene, which codes for GDP-mannose dehydrogenase, a key enzyme in the alginate biosynthetic pathway, is initiated at two promoters, one of which, p2, has sigma(E) consensus sequences. AlgU is the A. vinelandii alternative sigma(E) factor. In this study, we cons tructed an algU mutant (SMU88) which, as expected, is impaired in algi nate production, encystment, and transcription of the algD gene from t he p2 promoter. Plasmid pJMSAT1, carrying the A. vinelandii algU gene, restored alginate production and encystment to SMU88 and to strain UW 136, a naturally occurring algU mutant. Plasmid pSMU865, carrying the id. vinelandii mucABCD genes coding for negative regulators of AlgU ac tivity and previously shown to diminish alginate production in the wil d-type strain, ATCC 9046, was shown here to impair encystment and tran scription of the algD gene from the p2 algU-dependent promoter. Since nonencysting strain ATCC 9046/pSMU865 produced more alginate than some encysting strains, such as UW136/pJMSAT1, we propose an AlgU role in encystment, independent of the structural role that alginate plays in mature cysts.