Temporal and Spatial Co-expression of Hydrogenase and Nitrogenase Genes from Rhizobium leguminosarum bv. viciae in Pea (Pisum sativum L.) Root Nodules. Belen Brito. 'Laboratorio de Microbiologfa. Jose Manuel Palacios(1), Juan Imperial(2), T. Ruiz-Argiieso(1), Wei-Cai Yang(3), Ton Bisseling(3), Heidi Schmitt(4), Volkmar Kerl(4), Thorsten Bauer,4 Werner Kokotek(4), and Wolfgang Lotz(4) (1)Laboratorio de Microbiologfa; (2)C.S.I.C, ETS de Ingenieros Agronomos, Universidad Politecnica, E-28040 Madrid, Spain; (3)Department of Molecular Biology, Agricultural University, Dreijenlaan 3, NL-6703 HA Wageningen, The Netherlands; (4) Institut fur Mikrobiologie, Biochemie und Genetik, Universitat Erlangen-Niirnberg, Staudtstr. 5, D-91058 Erlangen, Germany. MPMI 8:235-240. Accepted 23 December 1994. Copyright 1995 The American Phytopathological Society.

Expression of the Rhizobium leguminosarum bv. viciae hydrogen uptake (hup) genes in pea root nodules has been studied by in situ hybridization. Accumulation of transcripts specific for the hydrogenase structural genes (hupSL) abruptly started at the beginning of the interzone II-III and continued throughout the nitrogen-fixing zone HI. This accumulation paralleled that of nifA- and nifH-specific mRNAs, and hupSL mRNAs were detected in the same cells as nif mRNAs. This suggests that the signals or regulators that trigger nitrogenase expression in nodules are also responsible for hydrogenase induction. Mutations in several hydrogenase accessory genes did not affect the pattern of hupSL expression, suggesting that these genes are not involved in hydrogenase regulation.