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RhoH is a member of the Rho family of small GTP-binding proteins that lacks GTPase activity. Since RhoH is constantly bound by GTP, it is thought to be constitutively active and controlled predominantly by changes in quantitative expression. RhoH is produced specifically in haematopoietic cells and aberrant expression has been linked to various forms of leukaemia. Transcription of the RHOH gene is the first level at which the quantitative levels of the RhoH protein are regulated. Previous studies have demonstrated that RHOH gene transcription is initiated by three distinct promoter regions designated P1, P2 and P3 that define the 5' end of exons 1, 2 and 4 respectively. In the present study we report that the P3 promoter is largely responsible for RHOH gene transcription in the B-lymphocytic cell line Raji. The P3 promoter contains a minimal promoter region and a repressor region extending from -236 to +67 and +68 to +245 respectively, relative to the 5' end of exon 4. Chromatin immunoprecipitation demonstrated that two AP1 (activator protein 1) sites in the minimal promoter region bind JunD. When JUND is overexpressed, the endogenous RHOH gene is repressed; however, when JUND is inhibited, expression of endogenous RHOH is induced both in the Raji cell line and AML (acute myeloid leukaemia) cells. In the HCL (hairy cell leukaemia) cell line JOK-1, induction of RHOH increases expression of the α isoform of protein kinase C. This downstream target of RHOH is also induced in AML cells by JUND inhibition. Collectively, these data indicate that JunD is an inhibitor of RHOH gene expression.

A human sialophorin (CD43) specific genomic clone was isolated, and a 6.5 kb fragment containing the 4.6 kb sialophorin gene was sequenced. The promoter region contains no TATA or CAAT boxes, but is highly enriched in G and C nucleotides and contains short repeat sequences similar to those found in the promoters of 'housekeeping' genes. S1-nuclease protection and primer-extension experiments established that the sialophorin gene has two major transcription initiation sites. There is a single intron of 378 bp that interrupts the sequence specifying the mRNA 5' untranslated region. The gene is therefore unusual in that the discrete extracellular, transmembrane and intracellular regions of the protein, including repeat sequences in the extracellular region, are not encoded by separate exons. Utilization of alternative polyadenylation signals was previously shown to generate two sialophorin mRNAs of 1.9 and 4.3 kb, which differ in the length of their 3' untranslated regions. Sequence analysis of the gene establishes that a single polyadenylation signal 2301 bp downstream of the first major transcription initiation site and five overlapping polyadenylation signals beginning a further 2290 bp downstream define the 3' termini of the 1.9 and 4.3 kb mRNA species respectively. The gene contains potential Z-DNA structures, Aly sequences, and elements that may be involved in regulating mRNA stability.