To more fully understand the biochemical and molecular events which occur in plants exposed to nitrate, cDNAs whose accumulation was enhanced in nitrate- and cycloheximide-treated maize (Zea mays L. W64A x W182E) roots were isolated. The 340 bp Zmrprn1 (for Zea mays root primary response to nitrate) cDNA also hybridized with a probe enriched for nitrate-induced sequences, and was characterized further. Sequence analysis of a near full-length cDNA (Zmrprn1A) showed strong homology (> 90% amino acid identity) with a root ferredoxin-NADP+ oxidoreductase (FNR) of rice, and 45-50% amino acid identify with leaf FNR genes. When expressed in Escherichia coli, the Zmrprn1A cDNA produced a protein with NADPH: ferricyanide reductase activity, consistent with the enzymatic properties of an FNR. The Zmrprn1 cDNA hybridized with a 1.4 kb transcript which was expressed in the maize root primary response to nitrate. That is, mRNA levels in roots increased rapidly and transiently in response to external nitrate, and low levels of nitrate (10 microM) induced transcript accumulation. The accumulation of the Zmrprn1 transcript was not prevented by cycloheximide, indicating that the cellular factor(s) required for expression were constitutively present in maize roots. The Zmrprn1 mRNA accumulated specifically in response to nitrate, since neither K+ nor NH4+ treatment of roots caused transcript accumulation. Maize leaves had about 5% of the transcript level found in roots, indicating a strong preference for expression of Zmrprn1 in roots. Analysis of maize genomic DNA indicated the presence of only a single gene or very small gene family for the Zmrprn1. Together, the data indicate that Zmrprn1A encodes a nitrate regulated maize root FNR.

A cluster of genes encoding subunits of ATP synthase of Anabaena sp. strain PCC 7120 was cloned, and the nucleotide sequences of the genes were determined. This cluster, denoted atpi, consists of four Fo genes and three F1 genes encoding the subunits a (atpl), c (atpH), b' (atpG), b (atpF), 8 (atpD), a (aptA), and y (atpC) in that order. Closely linked upstream of the ATP synthase subunit genes is an open reading frame denoted gene 1, which is equivalent to the uncI gene of Escherichia coli. The atp1 gene cluster is at least 10 kilobase pairs distant in the genome from apt2, a cluster of genes encoding the (atpB) and E (atpE) subunits of the ATP synthase. This two-clustered ATP synthase gene arrangement is intermediate between those found in chloroplasts and E. coli. A unique feature of the Anabaena atpl cluster is overlap between the coding regions for atpF and atpD. The atpl cluster is transcribed as a single 7-kilobase polycistronic mRNA that initiates 140 base pairs upstream of gene 1. The deduced translation products for the Anabaena sp. strain PCC 7120 subunit genes are more similar to chloroplast ATP synthase subunits than to those of E. coli.The proton-translocating ATP synthase is a multimeric membrane protein complex that couples a transmembrane gradient of electrochemical potential energy produced during electron transport to formation of ATP. This ubiquitous enzyme is found in cell membranes of bacteria, in inner mitochondrial membranes, and in thylakoid membranes of plant chloroplasts (reviewed in references 15, 18, and 30). In all examples studied the enzyme consists of two multimeric components: an extrinsic portion, F1, composed of subunits denoted a, P, -y, 8, and , and in integral membrane portion Fo, composed of several subunits which vary depending on the source of the ATP synthase. In Escherichia coli and chloroplasts, the two systems used for comparison in this study, there are three (a to c) and four (I to IV) Fo subunits, respectively. In E. coli, genes encoding all eight subunits of the ATP synthase are tightly linked and cotranscribed (15). For the ATP synthase of chloroplasts, genes for some subunits are encoded in the nucleus and others are encoded in the organelle genome. The chloroplast ATP synthase genes of higher plants are organized into two separate transcriptional units: the and E genes are linked and cotranscribed (35), while a second cluster containing the I, III, IV, and a. subunit genes map many kilobase pairs away in a second ATP synthase gene cluster (9, 19). The -y, 8, and subunit II genes are nuclear (34).The cyanobacteria are procaryotes with an oxygenevolving photosynthetic system nearly identical to that of plant chloroplasts. The similarity between cyanobacterial and plant photosystems, as well as the procaryotelike features of chloroplasts, lends support to the proposal (28) that * Corresponding author. t Paper no.

The cyanobacterium Anabaena PCC 7120 contains a multigene family that encodes the D1 polypeptide of Photosystem II. This family consists of four members denoted psbAI-IV that are each unique but highly homologous. psbAII, III and IV are more closely related to each other than to psbAI. These three copies encode identical polypeptides that differ from the psbAI product by 21 amino acids. The transcription initiation site for psbAI has been mapped to 64-65 nucleotides upstream from the coding region. Primer extension assays performed with an oligonucleotide specific for psbAII, III and IV transcripts suggest that one or more of these genes is also expressed. Genomic mapping and chromosome walking experiments demonstrate that none of the four psbA copies is within 20 kbp of another member of the gene family.

A family of repetitive elements from the cyanobacterium Anabaena sp. strain PCC 7120 was identified through the proximity of one element to the psbAI gene. Four members of this seven-member family were isolated and shown to have structures characteristic of bacterial insertion sequences. Each element is approximately 1,200 bp in length, is delimited by a 30-bp inverted repeat, and contains two open reading frames in tandem on the same DNA strand. The four copies differ from each other by small insertions or deletions, some of which alter the open reading frames. By using a system designed to trap insertion elements, one of the elements, denoted IS895, was shown to be mobile. The target site was not duplicated upon insertion of the element. Two other filamentous cyanobacterial strains were also found to contain sequences homologous to IS895.