. The absence of CD22 expression lowers the signalling threshold for BCR-crosslinking and can thus influence the fate of the B cell. We propose that the low threshold leads to hyperresponsiveness of the B cells and a chronic basal activation. In this model, engagement of the receptor without T-cell help leads to an increased induction of apoptosis, thus explaining the shorter lifespan of CD22(-/-) B cells and the low response to T-cell independent antigens. The alteration in B-cell phenotype and the higher levels of LPS-reactivity are attributable to the chronic basal stimulation.

Two different chalcone synthase (CHS) transcripts show similar expression characteristics under different light regimes in cotyledons of mustard (Sinapis alba L.). Etiolated seedlings show an increase in dark-expression 36-42 h after sowing. Under continuous red or far-red irradiation both CHS transcripts start to accumulate to levels above those of the dark control at 24-27 h after sowing. This time point can therefore be considered as the starting (or competence) point for phytochrome control of CHS. Continuous far-red irradiation stimulates transcript accumulation more than red light, indicating the involvement of a high-irradiance response (HIR). Irradiation of etiolated seedlings with 5 min long-wavelength far-red light (RG9) at 6-21 h after sowing decreases CHS-mRNA levels below those of the dark control. It is concluded that CHS dark-expression in etiolated seedlings is controlled by a pool of stabletype phytochrome which is derived from seed tissue. By contrast, an RG9-light pulse given to etiolated seedlings 30 h after sowing causes accumulation of CHS-mRNA above the dark-control level. This response and the HIR are attributed to the action of labile phytochrome for which the seedling becomes competent at the starting point 24-27 h after sowing. The different starting points for CHS-mRNA expression in darkness and in light (36 h and 24 h, respectively, after sowing) also indicate that the tested CHS genes in mustard are under the photocontrol of two distinct phytochrome pools. Northern analysis shows that both CHS-mRNAs are expressed in primary leaves, epicotyls and young flower buds. In-situ hybridization with gene-specific CHS probes reveals similar expression patterns for both transcripts in cotyledons of seedlings grown under 42 h continuous far-red light.

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