In U. maydis the multiallelic b locus controls sexual and pathogenic development. In the b locus a gene coding for a regulatory protein had been identified, and it was suggested that the interaction of two b polypeptides specified by different alleles programs sexual development in this fungus. We now demonstrate the existence of a second regulatory gene in the b locus. We term this gene bW and refer to the former as the bE gene. Both genes exist in many alleles. Although unrelated in primary sequence, both genes are similar in their overall organization. The gene products display allele-specific variability in their N-terminal domains, show a high degree of sequence conservation in the C-terminal domains, and contain a homeodomain-related motif. Genetic evidence is provided to show that the pair of bE and bW polypeptides encoded by different b alleles is the key regulatory species.

Internal ribosome entry sites (IRESs) facilitate an alternative, end-independent pathway of translation initiation. A particular family of dicistroviral IRESs can assemble elongation-competent 80S ribosomal complexes in the absence of canonical initiation factors and initiator transfer RNA. We present here a cryo-EM reconstruction of a dicistroviral IRES bound to the 80S ribosome. The resolution of the cryo-EM reconstruction, in the subnanometer range, allowed the molecular structure of the complete IRES in its active, ribosome-bound state to be solved. The structure, harboring three pseudoknot-containing domains, each with a specific functional role, shows how defined elements of the IRES emerge from a compactly folded core and interact with the key ribosomal components that form the A, P and E sites, where tRNAs normally bind. Our results exemplify the molecular strategy for recruitment of an IRES and reveal the dynamic features necessary for internal initiation.Initiation of protein synthesis is an essential phase of protein synthesis and a key regulatory step in gene expression 1,2 . In eukaryotes, the canonical 5¢ cap-dependent pathway is facilitated and orchestrated by approximately 11 translation initiation factors. However, IRES RNAs can functionally substitute for initiation factors and facilitate the alternative pathway of internal initiation 3,4 . IRESs are present in 5¢ untranslated regions (UTRs) of many viral RNAs and are efficient tools to hijack the translational apparatus of the host during viral infection. They are also used by a subset of cellular messenger RNAsfor example, several proto-oncogenes [3][4][5] . In this context, they act as regulatory tools and are used to initiate translation during cellular stress or other periods when overall global translation is compromised.The molecular mechanisms of initiation by IRES RNAs are largely unknown. IRES RNAs fall into different classes that are distinguished by their structure and dependence on different sets of canonical initiation factors and IRES trans-acting factors 3-6 . The simplest mechanism of initiation is used by the intergenic IRESs of dicistroviruses, such as the cricket paralysis virus (CrPV). This family of IRESs does not require any initiation factor or even initiator tRNA in order to assemble elongation-competent 80S ribosomes 7-10 . According to biochemical studies, the IRES binds directly to the ribosomal 40S subunit and sets the translational reading frame by positioning the first codon into the ribosomal A site. This is highly unusual, because canonical initiation starts from the P site. Moreover, like the hepatitis C virus IRES 11 , the CrPV IRES actively manipulates the conformation of the translational machinery, suggesting that the IRES acts like an RNA-based translation factor 12 .A detailed knowledge of the CrPV IRES structure, especially in the ribosome-bound state, is a prerequisite for understanding the mechanism of internal initiation without initiation factors. The low resolution of the previous cryo-EM maps limit...

SummaryResource allocation is a major determinant of plant ®tness and is in¯uenced by external as well as internal stimuli. We have investigated the effect of cell wall invertase activity on the transition from vegetative to reproductive growth, in¯orescence architecture, and reproductive output, i.e. seed production, in the model plant Arabidopsis thaliana by expressing a cell wall invertase under a meristem-speci®c promoter. Increased cell wall invertase activity causes accelerated¯owering and an increase in seed yield by nearly 30%. This increase is caused by an elevation of the number of siliques, which results from enhanced branching of the in¯orescence. On the contrary, as cytosolic enzyme, the invertase causes delayed¯ower-ing, reduced seed yield, and branching. This demonstrates that invertases not only are important in determining sink strength of storage organs but also play a role in regulating developmental processes.

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Using antipeptide antibodies with specificity for the carboxyl termini of v-raf and v-mil protein products, two proteins with apparent molecular weights of approximately 71,000/73,000 and 215,000 were detected in immunoprecipitates from normal uninfected chicken cells. The 71,000/73,000-molecular-weight protein was identified as the product of the c-mil proto-oncogene by the close structural relationship of its 42,000-molecular-weight carboxyl-terminal domain to the v-mil-encoded domain of the hybrid protein p100gag-mil specified by the avian retrovirus MH2. The amino-terminal domain of the cellular protein is encoded by 5' c-mil sequences that have not been transduced into the genome of MH2. The c-mil protein (p71/73c-mil) was found to be phosphorylated in vivo, and homologous proteins were detected at variable levels in a variety of vertebrate cells, including human cells.